Additive manufacturing process for producing ceramic articles using a sol containing nano-sized particles

ABSTRACT

The present invention relates to a process for producing a ceramic article, the process comprising the steps of providing a printing sol, the printing sol comprising solvent, nano-sized particles, radiation curable monomer(s) and photoinitiator, the printing sol having a viscosity of less than 500 mPa*s at 23° C., processing the printing sol as construction material in an additive manufacturing process to obtain a 3-dim article being in a gel state, the 3-dim article having a Volume A, transferring the 3-dim article being in a gel state to a 3-dim article being in an aerogel state, heat treating the 3-dim article to obtain a sintered 3-dim ceramic article, the ceramic article having a Volume F, Volume A of the 3-dim article in a gel state being more than 500% of Volume F of the ceramic article in its sintered state. The invention also relates to a ceramic article obtainable according to such a process. The ceramic article can have the shape of a dental or orthodontic article.

FIELD OF THE INVENTION

The invention relates to an additive manufacturing process for producingceramic articles using a sol containing nano-sized particles as aconstruction material. The invention also relates to ceramic articlesobtainable by such a process. The invention is particularly useful inthe dental and orthodontic area for producing dental restorations andorthodontic brackets.

BACKGROUND ART

In conventional ceramic processing, e.g. slip casting, the ceramicslurries usually have to have a particle load as high as possible toobtain an intermediate body with a high green density. A high greendensity is desired and needed to enable the production of dense sinteredceramics.

Powder-based additive manufacturing technologies where the low packingdensity of the powder bed results in a highly porous 3D object,typically does not result in a high density ceramic without the additionof large amounts of pressure during heat treatment, making therealization of dense complex three dimension shapes challenging.Typically this method leads to densities of less than 95% of thetheoretical density of the ceramic material.

The processing of slurries based on ceramic filled photopolymers withstereolithography has shown promise due to its ability to serve as agreen body in the production of relatively dense ceramic articles withthree dimensional architecture. Meanwhile, there are efforts trying toalso use additive manufacturing technologies (like SLA technology),which are mainly used for processing polymers, for the production ofceramic articles.

However, a high particle load in a slurry can be disadvantageous for therheological properties needed to process the slurry in an additivemanufacturing technique.

On the other hand, reducing the particle load in the slurry will resultin an article with a low green density which cannot be sintered to fulldensity without cracks.

It was also observed that the intermediate body resulting from theadditive manufacturing process is often not self-supporting.

U.S. Pat. No. 7,927,538 B2 (Moszner et al.) describes light-curing slipsfor the stereolithographic preparation of dental ceramics. The slipcomprises a polyreactive binder, photoinitiator, surface-modifiedceramic particles and a chain transfer agent. The viscosity of the sliplies in the range of 200 mPa*s to 2,000 Pa*s (23° C.).

U.S. Pat. No. 6,283,997M (Garg et al.) relates to a process forproducing a ceramic composite bone implant having a porous network froma photocurable polymer with a high volume percent of ceramiccomposition. For producing the photocurable ceramic composition, aluminaor hydroxyapatite having a particle size in the range of 0.05 to 10 μm(microns) is suggested.

U.S. Pat. No. 8,003,040 B2 (El-Siblani) relates to a process forproducing a 3-dim object by solidifying layers with electromagneticradiation of synergistic stimulation in a pattern.

US 2007/0072762 (Neil et al.) describes a method of making ceramicdischarge vessels for a lamp application using stereolithography. Theceramic-resin mixture used for this method contains a photocurableacrylate resin and ceramic powders like aluminum oxide, aluminumoxynitride, yttrium aluminum garnet and aluminum nitride powders havinga mean grain size in the range of d50=0.6 μm. The viscosity of themixture is in a range of 200 to 25,000 mPa·s.

U.S. Pat. No. 6,955,776 (Feenstra) relates to a method for making adental element by using a powder-based 3D printing technique. The powdercan be used in dry form or in dispersed in form (slurry). The powder canbe ceramic material or a metal. The ceramic material is preferablyselected from SiO₂, Al₂O₃, K₂O, Na₂O, CaO, Ba₂O, CrO₂, TiO₂, BaO, CeO₂,La₂O₃, MgO, ZnO and Li₂O. The powder used in the example has medianparticle size of d50: 0.5 to 0.7 μm.

US 2003/0222366 A1 (Stangel et al.) describes a dental restorationproduction in which a digitized optical impression of a dentalrestoration site is captured using an intra-oral camera. The capturedoptical impression is converted into a data file suitable forcomputer-assisted production using stereolithography. Theceramic-containing material should have a viscosity in the range of 200to 3.5 million centipoise (mPa*s). The mean particle size of the ceramicmaterial should be from 0.05 to 5 μm.

U.S. Pat. No. 8,133,831 (Laubersheimer et al.) describes a slip for thepreparation of dental ceramics by a hot-melt inkjet printing process.The slip contains ceramic particles, a radically polymerizable monomerand a wax. Ceramic particles based on Al2O3 or ZrO2 should have aparticle size of 50 to 500 nm (nanometers).

Similarly, US2012/308837 A1 (Schlechtriemen et al.) describes a processfor the generative preparation of shaped ceramic bodies by 3D inkjetprinting using different kinds of ceramic slips. The viscosity of theslips is said to be above 200 mPa*s at room temperature.

US 2014/0183799 A1 (Fischer et al.) deals with light-curing ceramicslips for the stereolithographic preparation of high-strength ceramicsusing a slip based on a radically polymerizable binder, polymerizationinitiator, filler and a certain acidic monomer comprising a radicallypolymerizable group. For Y-TZP zirconium dioxide, a particle size in therange of 50 to 3500 nm is said to be preferred. The rheologicalproperties of the slip are said to be in a range from 0.02 to 20,000Pa*s (23° C.).

U.S. Pat. No. 8,329,296 B2 (Apel et al.) relates to primary particles ofoxide-ceramic material having a primary particle size in the range of 10to 1,000 nm which are coated with a chromophoric component. Theparticles may be provided as a suspension comprising a polyreactivebinder, an organic solvent and additives. The suspension is said to havea viscosity from 200 to 2,000 Pa*s (23° C.).

WO 01/13815 A1 (Feenstra) describes a method for making a dental elementby a 3-dim printing technique. As curable material preferably ananomeric material consisting of nanomeric inorganic solid particleshaving polymerizable organic groups at their surface is used. After theprinting process, the dental element is typically subject to a thermalpost-treatment between 60 and 150° C. to complete curing. Insteadthereof, or supplemental thereof, a thermal densification isaccomplished wherein the dental element is heated to a temperature of atleast 250° C. However, the compositions described in the referencesabove have deficiencies.

Often, using a slurry or slip with ceramic particles greater than 50 nmin diameter is suggested. Not only are the slurry properties typicallynot suitable to produce highly accurate ceramic articles, but the largerparticle sizes, even when closely packed, still limit the percentage oftheoretical density possible, limiting the final material propertiesincluding mechanical as well as optical performance. Thus, there is aneed for an improved additive manufacturing process.

There is also a need for high strength, translucent printed article,preferably a high strength, translucent printed zirconia article.

DESCRIPTION OF THE INVENTION

It is an object of the invention to improve existing additivemanufacturing processes. In particular, it is an object of the inventionto provide a process allowing the production of ceramic articles with adensity close to theoretical density, high strength, high accuracyand/or good translucency.

In particular, an additive manufacturing process is desirable allowingthe production of ceramic articles with good surface quality.

This object can be achieved by a process as described in the text below.

Such a process comprises the steps of:

-   -   providing a printing sol, the printing sol comprising solvent,        nano-sized particles, radiation curable component(s) and        photoinitiator, the printing sol having a viscosity below 500 or        below 200 mPa*s (23° C.),    -   processing the printing sol as construction material in an        additive manufacturing process to obtain a 3-dim article being        in a gel state, the 3-dim article having a Volume A,    -   transferring the 3-dim article being in a gel state to a 3-dim        article being in a dry state, preferably an aerogel or xerogel,    -   applying a heat treatment step to obtain a sintered 3-dim        ceramic article, the ceramic article having a Volume F,    -   Volume A of the 3-dim article in a gel state being more than        200% or more than 300% or more than 400% or more than 500% of        Volume F of the ceramic article in its sintered state.

The invention is also directed to a ceramic article obtainable orobtained according to a process as described in the present text, theceramic article being characterized by at least one, more or all of thefollowing features:

-   -   density: more than 98.5% with respect to theoretical density;    -   translucency: more than 30% determined on a polished sample        having a thickness of 1 mm;    -   biaxial flexural strength: at least 450 MPa according to ISO        6872;    -   dimension in either x, y or z direction: at least 0.25 mm.

Definitions

“Ceramic” or “ceramic article” means a non-metallic material that isproduced by application of heat. Ceramics are usually hard, and brittleand, in contrast to glasses or glass-ceramics, display an essentiallypurely crystalline structure. Ceramics are usually classified asinorganic materials.

“Crystalline” means a solid composed of atoms arranged in a patternperiodic in three dimensions (i.e., has long-range crystal structurewhich may be determined by techniques such as X-ray diffraction).

A “crystallite” means a crystalline domain of a solid having a definedcrystal structure. A crystallite can only have one crystal phase.

“Additive manufacturing” means processes used to make 3-dimensionalarticles. An example of an additive manufacturing technique isstereolithography (SLA) in which successive layers of material are laiddown under computer control. The articles can be of almost any shape orgeometry and are produced from a 3-dimensional model or other electronicdata source.

The term “dental or orthodontic ceramic article” means any ceramicarticle which is to be used in the dental or orthodontic field,especially for producing a dental restoration, orthodontic devices, atooth model and parts thereof.

Examples of dental articles include crowns, bridges, inlays, onlays,veneers, facings, copings, crown and bridged framework, implants,abutments, dental milling blocks, monolithic dental restorations andparts thereof.

Examples of orthodontic articles include brackets, buccal tubes, cleatsand buttons and parts thereof.

A dental or orthodontic article should not contain components which aredetrimental to the patient's health and thus free of hazardous and toxiccomponents being able to migrate out of the dental or orthodonticarticle. The surface of a tooth is considered not to be a dental ororthodontic article.

“Zirconia article” shall mean a 3-dimensional (3-dim) article wherein atleast one of the x, y, z dimensions is at least 1 mm, at least 0.5 mm,or at least 0.25 mm, the article being comprised of at least about 80 orat least about 85 or at least about 90 or at least about 95 wt.-%zirconia.

“Monolithic dental restoration” shall mean a dental ceramic article ontothe surface of which no facing or veneer has been attached. That is, themonolithic dental restoration is essentially comprised out of only onematerial composition. However, if desired a thin glazing layer can beapplied.

“Glass” means an inorganic non-metallic amorphous material which isthermodynamically an under-cooled and frozen melt. Glass refers to ahard, brittle, transparent solid. Typical examples include soda-limeglass and borosilicate glass. A glass is an inorganic product of fusionwhich has been cooled to a rigid condition without crystallizing. Mostglasses contain silica as their main component and a certain amount ofglass former. The material or article described in the present text doesnot contain a glass.

“Glass-ceramic” means an inorganic non-metallic material where one ormore crystalline phases are surrounded by a glassy phase so that thematerial comprises a glass material and a ceramic material in acombination or mixture. Thus, a glass-ceramic is a material sharing manyproperties with both glass and more traditional crystalline ceramics. Itis formed as a glass, and then made to crystallize partly by heattreatment. Glass-ceramics may refer to a mixture of lithium-, silicon-and aluminum oxides.

The material or article described in the present text does not contain aglass-ceramic.

“Sol” refers to a continuous liquid phase containing discrete particleshaving sizes in a range from 1 nm to 100 nm or from 1 nm to 50 nm, a socalled “colloidal solution”. The sols described in the present text aretranslucent and do show a so-called “Tyndall effect” or “Tyndallscattering”. The size of the particles is below the wavelength of thevisible light (400 to 750 nm).

A transparent material lets light pass through according to Snell's law(classical law of refraction). So, a picture can be seen in its detailsthrough a platelet of a transparent material.

A translucent material lets light partially permeate through although itis not fully transparent, i.e. showing a significant volume scatteringof the transmitted light. The reciprocal property of translucency isopacity (O). O=1/T=I/I0 (T=Transmission, I=Intensity of permeated light,I=Intensity of light before permeation). So, opacity values less thanabout 0.9 for a 1 mm thick platelet with a diameter of 15 mm areregarded as translucent (e.g. for a measurement with a Color i7 device,X-Rite corporation USA, measurement mode: remission contrast ratio).Opacity can be measured by various means: in transmission, in remission,and in remission using the contrast ratio method.

By “machining” is meant milling, grinding, cutting, carving, or shapinga material by a machine. Milling is usually faster and more costeffective than grinding. A “machinable article” is an article having a3-dimensional shape and having sufficient strength to be machined.

A “powder” means a dry, bulk material composed of a large number of fineparticles that may flow freely when shaken or tilted.

A “particle” means a substance being a solid having a shape which can begeometrically determined. The shape can be regular or irregular.Particles can typically be analysed with respect to e.g. particle sizeand particle size distribution. A particle can comprise one or morecrystallites. Thus, a particle can comprise one or more crystal phases.

The term “associated” refers to a grouping of two or more primaryparticles that are aggregated and/or agglomerated. Similarly, the term“non-associated” refers to two or more primary particles that are freeor substantially free from aggregation and/or agglomeration.

The term “aggregation” refers to a strong association of two or moreprimary particles. For example, the primary particles may be chemicallybound to one another. The breakdown of aggregates into smaller particles(e.g., primary particles) is generally difficult to achieve.

The term “agglomeration” refers to a weak association of two or moreprimary particles. For example, particles may be held together by chargeor polarity. The breakdown of agglomerates into smaller particles (e.g.,primary particles) is less difficult than the breakdown of aggregatesinto smaller particles.

The term “primary particle size” refers to the size of a non-associatedsingle crystal zirconia particle, which is considered to be a primaryparticle. X-ray diffraction (XRD) is typically used to measure theprimary particle size.

“Soluble” means that a component (e.g. solid) can be completelydissolved within a solvent. That is, the substance is able to formindividual molecules (like glucose) or ions (like sodium chloride) ornon-settling particles (like a sol) when dispersed in water at 23° C.The solubilisation process, however, might take some time, e.g. stirringthe component over a couple of hours (e.g. 10 or 20 h) might berequired.

“Density” means the ratio of mass to volume of an object. The unit ofdensity is typically g/cm³. The density of an object can be calculatede.g. by determining its volume (e.g. by calculation or applying theArchimedes principle or method) and measuring its mass.

The volume of a sample can be determined based on the overall outerdimensions of the sample. The density of the sample can be calculatedfrom the measured sample volume and the sample mass. The total volume ofa material sample can be calculated from the mass of the sample and thedensity of the used material. The total volume of cells in the sample isassumed to be the remainder of the sample volume (100% minus the totalvolume of material).

A “porous material” refers to a material comprising a partial volumethat is formed by voids, pores, or cells in the technical field ofceramics. Accordingly an “open-celled” structure of a material sometimesis referred to as “open-porous” structure, and a “closed-celled”material structure sometimes is referred to as a “closed-porous”structure. It may also be found that instead of the term “cell”sometimes “pore” is used in this technical field. The material structurecategories “open-celled” and “closed-celled” can be determined fordifferent porosities measured on different material samples (e.g. usinga mercury “Poremaster 60-GT” from Quantachrome Inc., USA) according toDIN 66133. A material having an open-celled or open-porous structure canbe passed through by e.g. gases.

The “average connected pore diameter” means the average size of theopen-celled pores of a material. The average connected pore diameter canbe calculated as described in the Examples section.

The term “calcining” or “debindering” refers to a process of heatingsolid material to drive off at least 90 percent by weight of volatilechemically bond components (e.g., organic components) (vs., for example,drying, in which physically bonded water is driven off by heating).Calcining is done at a temperature below a temperature needed to conducta pre-sintering step.

The terms “sintering” or “firing” are used interchangeably. Apre-sintered ceramic article shrinks during a sintering step, that is,if an adequate temperature is applied. The sintering temperature to beapplied depends on the ceramic material chosen. For ZrO₂ based ceramicsa typical sintering temperature range is about 1100° C. to about 1550°C. Sintering typically includes the densification of a porous materialto a less porous material (or a material having less cells) having ahigher density, in some cases sintering may also include changes of thematerial phase composition (for example, a partial conversion of anamorphous phase toward a crystalline phase).

“Diafiltration” is a technique that uses ultrafiltration membranes tocompletely remove, replace, or lower the concentration of salts orsolvents from solutions containing organic molecules. The processselectively utilizes permeable (porous) membrane filters to separate thecomponents of solutions and suspensions based on their molecular size.

“Green body gel” means a three-dim gel resulting from the curingreaction of polymerizable components contained in a sol, includingorganic binder and solvent.

“Aerogel” means a three-dimensional low-density (e.g., less than 20% oftheoretical density) solid. An aerogel is a porous material derived froma gel, in which the liquid component of the gel has been replaced with agas. The solvent removal is often done under supercritical conditions.During this process the network does not substantially shrink and ahighly porous, low-density material can be obtained.

“Xerogel” refers to a three-dimensional solid derived from a green bodygel, in which the liquid component of the gel has been removed byevaporation under ambient conditions or at an elevated temperature.

A “green body” means an un-sintered ceramic item, typically having anorganic binder present.

A “white body” means a pre-sintered ceramic item.

A “geometrically defined article” means an article the shape of whichcan be described with geometrical terms including 2-dimensional termslike circle, square, rectangle, and 3-dimensional terms like layer,cube, cuboid, sphere.

“Isotropic linear sintering behaviour” means that the sintering of aporous body during the sintering process occurs essentially invariantwith respect to the directions x, y and z.

“Essentially invariant” means that the difference in sintering behaviourwith respect to the directions x, y and z is in a range of not more thanabout +/−5% or +/−2% or +/−1%.

A material or composition is “essentially or substantially free of” acertain component within the meaning of the invention, if the materialor composition does not contain said component as an essential feature.Thus, said component is not willfully added to the composition ormaterial either as such or in combination with other components oringredient of other components. A composition or material beingessentially free of a certain component usually contains the componentin an amount of less than about 1 wt.-% or less than about 0.1 wt.-% orless than about 0.01 wt.-% (or less than about 0.05 mol/l solvent orless than about 0.005 mol/l solvent or less than about 0.0005 mol/lsolvent) with respect to the whole composition or material. Ideally thecomposition or material does not contain the said component at all.However, sometimes the presence of a small amount of the said componentis not avoidable e.g. due to impurities.

As used herein, “a”, “an”, “the”, “at least one” and “one or more” areused interchangeably. The terms “comprises” or “contains” and variationsthereof do not have a limiting meaning where these terms appear in thedescription and claims. Also herein, the recitations of numerical rangesby endpoints include all numbers subsumed within that range (e.g., 1 to5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

Adding an “(s)” to a term means that the term should include thesingular and plural form. E.g. the term “additive(s)” means one additiveand more additives (e.g. 2, 3, 4, etc.). Unless otherwise indicated, allnumbers expressing quantities of ingredients, measurement of physicalproperties such as described below and so forth used in thespecification and claims are to be understood as being modified in allinstances by the term “about”.

The term “comprise” shall include also the terms “consist essentiallyof” and “consists of”.

BRIEF DESCRIPTION OF FIGURES

FIG. 1A to 1D show the development of shrinkage from a printed sample tothe final ceramic article after sintering.

FIG. 2 compares the size of a printed gel-like cuboid with the size ofthe sintered cuboid ceramic article resulting from this cuboid.

FIG. 3 schematically shows a device for conducting a stereolithographyprocess (SLA) using the bottom-up projection technique.

DETAILED DESCRIPTION OF THE INVENTION

It was found that the process described in the present text is inparticular useful for producing ceramic articles with a high surfaceaccuracy.

Processing the sol to obtain a 3-dim article with a volume being muchlarger than the volume of the ceramic article after sinteringfacilitates the production of ceramic articles with high accuracy andsurface smoothness.

The larger the enlargement factor for producing the 3-dim article in agel state is, the smaller the surface defects on the ceramic articleafter sintering will be.

Visible surface defects which may occur during the additivemanufacturing process and which may be caused by limited resolution ofthe manufacturing equipment, will shrink during the sintering process bythe enlargement factor chosen.

However, the enlargement factor cannot be chosen freely as the 3-dimarticle obtained after the additive manufacturing process needs to befree-standing. That is, the 3-dim article needs to have a sufficientconsistency and stability allowing it to be removed from the additivemanufacturing equipment without distortion or destruction.

It was found that the sol described in the present text facilitates theproduction of 3-dim articles in a gel state by applying additivemanufacturing techniques, wherein the 3-dim articles can be sintered toceramic articles without cracks afterwards.

The printing sol typically contains a sufficient high amount ofnano-sized particles to produce a free-standing 3-dim article in a gelstate, even if the dimension of the 3-dim article in the gel state isenlarged by at least 50% or more compared to the final ceramic articlein its sintered state.

On the other hand, the content of nano-sized particles is sufficientlylow to produce a 3-dim article in its gel state which can be sinteredafterwards without cracks. The amount of nano-sized particles beingpresent in the sol is related to its viscosity.

Increasing the volume content of the nano-sized particles wouldfacilitate the production of an enlarged free-standing 3-dim article inits gel state. However, such a 3-dim article cannot be sintered to itsdesired size and shape without cracks. The content of the nano-sizedparticles will be too high.

If on the other hand, the content of the nano-sized particles is toolow, producing an enlarged free-standing 3-dim article in its gel statewill not be possible.

It was found that a sol as described in the present text is highlysuitable as construction material for producing ceramic articlesaccording to the process described in the present text.

Compared with a subtractive processing technique, the application of anadditive manufacturing technique in addition provides for a significantreduction in material usage and potentially a significant reduction inburnout and/or sintering time due to the smaller part size.

Once sintered, the ceramic articles show good physical properties likehigh bending strength and/or fraction resistance.

Further, the ceramic articles are sufficient translucent and can thus beused in particular in the dental and orthodontic field.

The ceramic articles can be produced with high accuracy in particular asregards details or smoothness of the surface.

During the additive manufacturing process there is no need to apply heatin order to make the construction material sufficient fluidly in orderto process them through nozzles.

There is also no need to add further additives like waxes to theconstruction material needed for stabilizing the 3-dim article obtainedafter conducting the additive manufacturing technique.

The invention facilitates an additive, mould-free approach to final partfabrication as compared to existing moulding and/or subtractiveprocesses, e.g. machining a ceramic article out of a mill blank.

By manufacturing the intermediate body in an enlarged stage may alsoallow for an enhanced processing speed as the requirement for a highsurface resolution during the manufacturing process is reduced. Surfacedefects will disappear or be minimized during the shrinking processoccurring during the sintering step.

Furthermore, another positive aspect of the use of a sol-gel basedprocess with high shrinkage factors is, that it allows to finishfunctional surfaces or features by a subtractive process (e.g. millingetc.) or a finishing process (e.g. polishing, etc.) in the enlarged gelstate to leverage the shrinkage effect for very precise details andsmooth surfaces. This could additionally be feasible for increasedmechanical properties (reduction of surface flaws).

The process described in the present test allows the production ofsol-gel bodies with a resolution of 25 μm in z-direction and 40 μm×40 μmin x-y direction (=pixelsize) resulting in a ceramic body (aftersintering) with a resolution of e.g. 12.5 μm in z-direction and 20×20 μmin x-y direction.

The invention is now described in further details:

According to the process described in the present text, a printing solcomprising nano-sized particles is provided.

The printing sol is processed as construction material in an additivemanufacturing process.

The processing of the printing sol results in a 3-dim article being in agel state. This 3-dim article has a Volume A.

The 3-dim article is transferred into a 3-dim article being in a drystate, preferably in an aerogel or xerogel state.

The 3-dim article being in a dry state is heat treated to obtain asintered 3-dim article having a Volume F.

During the heat treatment the 3-dim article undergoes a volume shrinkageof more than 50% or more than 200% or more than 300% or more than 400%or more than 500%.

Additive manufacturing techniques which can be used includestereolithographic printing.

According to another embodiment, the process comprises the followingsteps:

-   (a) providing a printing sol comprising nano-sized particles,    solvent, radiation curable monomer(s), photoinitiator, optionally    organic dye(s) and optionally inhibitor(s), as described in the    present text,-   (b) processing the printing sol as construction material in an    additive manufacturing process to obtain a 3-dim article being in a    gel state, the 3-dim article having a Volume A,-   (c) additive manufacturing the desired geometries by light curing,-   (d) optionally cleaning the surface of the 3-dim article being in a    gel state, in particular for the purpose of removing residues of    non-reacted sol,-   (e) optionally postcuring (e.g. by heating or lightcuring) the 3-dim    article being in a gel state to a temperature in the range of 35 to    80° C. or by additional light curing, in particular for the purpose    of increasing the stability of the 3-dim article being in a gel    state, that 3-dim article having a Volume B,-   (f) optionally soaking the 3-dim article being in a gel state with    another solvent (e.g. diethylene glycol ethyl ether or ethanol),-   (g) transferring the 3-dim article being in a gel state to a 3-dim    article being in a dry state, preferably by applying a supercritical    drying step to the 3-dim article being in a gel state, in particular    for the purpose of removing the solvent, that 3-dim article having a    Volume C,-   (h) optionally heating the 3-dim article being in a dry state to a    temperature in the range of 400 to 800° C., in particular for the    purpose of removing residual organic components and to further    increasing the stability, to obtain a green body, that 3-dim article    having a Volume D,-   (i) optionally heating the 3-dim article of the previous step to a    temperature in the range of 800 to 1100° C., in particular for the    purpose of creating a pre-sintered body or white body having a    porous structure, that 3-dim article having a Volume E,-   (j) optionally coloring at least parts of the surface of the 3-dim    article of the previous step, in particular for the purpose of    adjusting the color and individualizing the desired 3-d article,-   (k) applying a heat treatment step to obtain a sintered 3-dim    ceramic article, the ceramic article having a Volume F,    -   wherein Volume A=or >Volume B>Volume C>Volume D>Volume E>Volume        F.

The symbol “=” means that Volume A is essentially equal to Volume B.Thus, a slight deviation of e.g. +/−5% is allowed.

If desired and if a printing sol is used which already contains coloringcomponents, steps (h), (i), (j) and (k) can be combined.

Similarly, if no coloring is desired, step (j) can be omitted and steps(h), (i) and (k) can be done within one step. Thus, there is no need orrequirement to conduct those steps separately.

By continuously heating the 3-dim article being in an aerogel state to atemperature up to 1000 or 1050 or 1100° C., the organic residues will beburnt out first before the remaining inorganic components begin toadhere together and form a pre-sintered article.

According to one embodiment, the relationship of the individual volumesis as follows, for Volume A being scaled to 100:

Volume A=100

Volume B=100 to 99 or 100 to 99.5 or 100 to 99.9

Volume C=90 to 50

Volume D=70 to 25 or

Volume E=35 to 15

Volume F=25 to 2.

That is, compared to the 3-dim article being in a gel state, the 3-dimarticle in an aerogel state typically shows volume shrinkage in therange of 0 to 50%.

Compared to the 3-dim article being in a gel state, the 3-dim article ina pre-sintered state typically shows volume shrinkage in the range of 25to 75%.

Compared to the 3-dim article being in a gel state, the 3-dim article ina sintered state typically shows volume shrinkage in the range of 75 to98%.

According to one embodiment, the relationship of the individual volumesis as follows, for Volume F being scaled to 100:

Volume A (gel body)=850 to 1250 or 900 to 1100.

Volume B=850 to 1250 or 900 to 1100.

Volume C (aerogel body)=600 to 900 or 650 to 850

Volume D=250 to 700 or 300 to 600

Volume E (pre-sintered)=140 to 360 or 150 to 250

Volume F (fully sintered)=100.

According to one embodiment, Volume A of the 3-dim article being in agel state is more than 200% or more than 300% or more than 500% or morethan 800% or more than 900% of Volume F of the ceramic article being inits sintered state.

Processing the Sol

The printing sol described in the present text is processed in anadditive manufacturing process, in particular in a stereolithographyprocess (SLA).

According to one embodiment, in the process of producing a ceramicarticle as described in the present text, the processing step comprisesthe steps of

-   -   providing a layer of the construction material on a surface,    -   radiation curing those parts of the layer of construction        material which will belong to the 3-dim article to be produced,    -   providing an additional layer of the construction material in        contact with the radiation cured surface of the previous layer,    -   repeating the previous steps until a 3-dim article is obtained.

Such a process comprises the step of applying radiation to the surfaceof a radiation curable material, wherein the radiation is applied onlyto those parts of the surface which will later form a part of thearticle to be produced.

Radiation can be applied by using e.g. a laser beam or by mask-imageprojection. Using a mask-image projection based stereolithographieprocess (MIP-SL) is sometimes preferred, as it allows a more rapidmanufacturing of the article.

A MIP-SL process can be described as follows:

-   i. A three-dimensional digital model of the article to be produced    is provided.-   ii. The three-dimensional digital model is sliced by a set of    horizontal planes.-   iii. Each thin slice is converted into a two-dimensional mask image.-   iv. The mask image is then projected with the aid of a radiation    source onto the surface of the radiation curable material being    located in a building platform (e.g. having the shape of a vat).-   v. The radiation curable material is only cured in those regions    which are exposed.-   vi. The building platform containing the radiation curable material    or the layer of cured material is moved relative to the radiation    source, wherein a new layer of radiation curable material is    provided being in contact with the layer of the cured material    produced in the previous step.-   vii. Steps (iv) to (vi) are repeated until the desired article is    formed.

Projecting the mask image on the radiation curable material can be doneeither top-down or bottom-up with respect to the orientation of the vat.

Using the bottom-up technique can be beneficial as less radiationcurable material is needed.

In this process, the radiation cured layer is formed on the bottom ofthe vat, which is transparent. FIG. 3 schematically shows how such abottom-up technique works. (1) is the light source, (2) is a mirror, (3)is a lens, (4) is the building platform having the shape of a vat, (5)is the radiation-curable construction material being located in thebuilding platform, (6) is a motor connected with a leadscrew (7).Attached to the leadscrew (7) is a holder (8). Holder (8) is used forholding the 3-dim article to be produced.

It was found that the printing sol described in the present text is inparticular useful for processing it in a mask-image projectionstereolithography process using the bottom-up projection technique.

Further details of such a processing step are described in U.S. Pat. No.4,575,330 (Hull), U.S. Pat. No. 6,283,997 (Garg et al.) or U.S. Pat. No.8,003,040 B2 (El-Siblani). The content of these documents is herewithincorporated by reference.

The processing of the printing sol can be done by using or applying atleast one or more of the following parameters:

-   -   Slice thickness of printing sol exposed to radiation: 0.001 to        0.500 mm or 0.01 to 0.4 mm;    -   Energy dose per layer in the range of 5 mJ/cm² to 100 mJ/cm² or        8 mJ/cm² to 50 mJ/cm².

The printing sol containing the nano-sized particles is solidified bygelation.

Preferably, the gelation process allows green body gels to be formed ofany shape without cracks and green body gels that can be furtherprocessed without inducing cracks. For example, preferably, the gelationprocess leads to a green body gel having a structure that will notcollapse when the solvent is removed; so-called “free-standing gel”. Thegreen body gel structure is compatible with and stable in a variety ofsolvents and conditions that may be necessary for supercriticalextraction. Furthermore, the gel structure should be compatible withsupercritical extraction fluids (e.g., supercritical CO₂). In otherwords, the gels should be stable and strong enough to withstand drying,so as to produce stable aerogels and give materials that can be heatedto burn out the organics, pre-sintered, and densified without inducingcracks. Preferably, the resulting aerogels have relatively small anduniform pore sizes to aid in sintering them to high density at lowsintering temperatures. However, preferably the pores of the aerogelsare large enough to allow product gases of organic burnout to escapewithout leading to cracking of the aerogel. It is believed that therapid nature of the gelation step results in an essentially homogeneousdistribution of the zirconia-based particles throughout the gel, whichcan aid in the subsequent processing steps such as supercriticalextraction, organic burnout, and sintering. It is preferable that thegel contain the minimum amount of organic material or polymer modifiers.

After processing the printing sol to form a green body gel, the 3-dimarticle in its gel-state is typically removed from the device used forconducting the additive manufacturing process.

If desired, the surface of the 3-dim article in its gel-state iscleaned, e.g. by rinsing the 3-dim article with a solvent or soaking ina solvent.

Suitable solvents preferably include mixtures thereof or the samesolvent(s) used in the sol described in the present text.

Suitable solvents include either low boiling alcohols as described inthe present text (e.g. an alcohol having a boiling point below 100° C.;like methanol, ethanol, propanol) and mixtures thereof or high boilingsolvents as described in the present text, preferably the samesolvent(s) being present in the sol, e.g. diethylene glycol ethyl ether.

If desired, the 3-dim article in its gel-state can be post-cured byapplying radiation or heat.

Such a step may help to improve the stability of the 3-dim article inits gel-state by further increasing the degree of polymerization.

If applied, the post-curing step can be characterized by at least one,or all of the following features:

-   -   Applying radiation with wavelength from 200 to 500 or from 350        to 450 nm;    -   Applying a heating step with a temperature below the temperature        at which drying will occur or which is used for debindering or        calcining; e.g. from 30 to 110 or from 40 to 80° C.

The process of producing the ceramic article described in the presenttext typically also comprises a drying step to remove any organicsolvents or water that may be present, transferring the 3-dim articlebeing in a gel state to a 3-dim article being in a dry state. This canbe referred to as drying the green gel body or the printed gel articleregardless of the method used to remove the organic solvent.

In some embodiments, removal of the organic solvent occurs by drying theprinted gel article at room temperature (e.g., 20° C. to 25° C.) or atan elevated temperature. Any desired drying temperature up to 200° C.can be used. If the drying temperature is higher, the rate of organicsolvent removal may be too rapid and cracking can result. A xerogelresults from this process of organic solvent removal.

Forming a xerogel can be used for drying printed gel articles with anydimensions, but is most frequently used for the preparation ofrelatively small sintered articles. As the gel composition dries, eitherat room temperature or at elevated temperatures, the density of thestructure increases. Capillary forces pull the structure togetherresulting in some linear shrinkage such as up to about 30%, up to 25% orup to 20%. The shrinkage is typically dependent on the amount ofinorganic oxide present and the overall composition. The linearshrinkage is often in a range of 5 to 30%, 10 to 25%, or 5 to 15%.Because the drying typically occurs most rapidly at the outer surfaces,density gradients are often established throughout the structure.Density gradients can lead to the formation of cracks. The likelihood ofcrack formation increases with the size and the complexity of theprinted gel article and with the complexity of the structure. In someembodiments, xerogels are used to prepare sintered bodies having alongest dimension no greater than about 1 centimeter.

In some embodiments, the xerogels contain some residual organic solvent.The residual solvent can be up to 6 wt.-% based on the total weight ofthe xerogel. For example, the xerogel can contain up to 5 wt.-%, up to 4wt.-%, up to 3 wt.-%, up to 2 wt.-%, or up to 1 wt.-% organic solvent.

If the printed gel article has fine features that can be easily brokenor cracked, it is often preferable to form an aerogel intermediaterather than a xerogel. A printed gel article of any size and complexitycan be dried to an aerogel. An aerogel can be formed by drying theprinted gel article, preferably under supercritical conditions. There isno capillary effect for this type of drying, and the linear shrinkage isoften in a range of 0 to 25%, 0 to 20%, 0 to 15%, 5 to 15%, or 0 to 10%.The density typically remains uniform throughout the structure.

If applied, the supercritical drying step can be characterized by atleast one, more or all of the following features:

a) Temperature: 20 to 100° C. or 30 to 80° C. or 15 to 150° C.;

b) Pressure: 5 to 200 MPa or 10 to 100 MPa or 1 to 20 MPa or 5 to 15MPa;

c) Duration: 2 to 175 h or 5 to 25 h or 1 to 5 h;

d) Extraction or drying medium: carbon dioxide in its supercriticalstage,

A combination of features (a), (b) and (d) is sometimes preferred.

Supercritical extraction can remove all or most of the organic solventin the printed gel article. In some embodiments, the aerogels containsome residual organic solvent. The residual solvent can be up to 6 wt.-%based on the total weight of the aerogel. For example, the aerogel cancontain up to 5 wt.-%, up to 4 wt.-%, up to 3 wt.-%, up to 2 wt.-%, orup to 1 wt.-% organic solvent.

The removal of organic solvent results in the formation of pores withinthe dried structure. Preferably, the pores are sufficiently large toallow gases from the decomposition products of the polymeric material toescape without cracking the structure when the dried structure isfurther heated to burnout the organic material and to form a sinteredarticle.

The article obtained after having conducted the supercritical dryingstep can typically be characterized by at least one or more of thefollowing properties:

-   -   showing a N₂ adsorption and/or desorption isotherm with a        hysteresis loop;    -   showing a N₂ adsorption and desorption of isotherm type IV        according to IUPAC classification and a hysteresis loop;    -   showing a N₂ adsorption and desorption isotherm of type IV with        a hysteresis loop of type H1 according to IUPAC classification;    -   showing a N₂ adsorption and desorption isotherm of type IV with        a hysteresis loop of type H1 according to IUPAC classification        in a p/p0 range of 0.70 to 0.99;    -   BET surface: from 120 to 200 m²/g or from 130 to 190 m²/g.

A heating step is conducted to remove organic residues being stillpresent in the 3-dim article before final sintering. Removing organicresidues before sintering reduces the risks of cracks during sintering.

Such a heating step is sometimes also referred to as debindering orcalcining step. As a result a so-called “white body” is obtained.

The heating step is typically conducted at a temperature below 800° C.or below 700 or below 600° C. A typical temperature range is from 400 to800° C. or from 500 to 700° C.

The heating step is typically conducted for a time needed to combust theorganic components in the 3-dim article.

A typical time frame is from 5 to 100 h or from 10 to 50 h.

The heating is typically conducted at ambient conditions (i.e. ambientair, ambient pressure).

The body obtained after a debindering or calcining step can be furthertreated with heat to obtain a pre-sintered article.

Such a pre-sintered article is porous and can be colored usingcommercially available coloring liquids. If either no coloring isdesired or if the article is already colored, a pre-sintering step canbe omitted.

The pre-sintered article can typically be characterized by the followingproperties:

-   -   showing a N₂ adsorption and/or desorption isotherm with a        hysteresis loop;    -   showing a N₂ adsorption and desorption of isotherm type IV        according to IUPAC classification and a hysteresis loop;    -   showing a N₂ adsorption and desorption isotherm of type IV with        a hysteresis loop of type H1 according to IUPAC classification;    -   showing a N₂ adsorption and desorption isotherm of type IV with        a hysteresis loop of type H1 according to IUPAC classification        in a p/p0 range of 0.70 to 0.99;    -   BET surface: from 15 to 100 m²/g or from 16 to 60 m²/g.

The conditions to be applied for conducting a pre-sintering step can bedescribed as follows:

-   -   temperature: from 800 to 1100° C. or from 950 to 1090° C. or        from 975 to 1080° C.;    -   atmosphere: air or inert gas (e.g. nitrogen, argon);    -   duration: until a density of 40 to 60% of the final density of        the material has been reached.

If desired, the pre-sintered article optionally can be soaked in a basicsolution such as an aqueous solution of ammonium hydroxide. Soaking canbe effective to remove undesirable ionic species such as sulfate ionsbecause of the porous nature of the articles at this stage of theprocess. Sulfate ions can ion exchange with hydroxyl ions. If sulfateions are not removed, they can generate small pores in the sinteredarticles that tend to reduce the translucency and/or the strength.

More specifically, the ion exchange process often includes soaking thearticle that has been heated to remove organic material in an aqueoussolution of 1 N ammonium hydroxide. This soaking step is often for atleast 8 h, at least 16 h, or at least 24 h. After soaking, the articleis removed from the ammonium hydroxide solution and washed thoroughlywith water. The article can be soaked in water for any desired period oftime such as at least 30 min, at least 1 h, at least 2 h, or at least 4h. The soaking in water can be repeated several times, if desired, byreplacing the water with fresh water.

After soaking, the article is typically dried in an oven to remove thewater. For example, the article can be dried by heating in an oven setat a temperature equal to at least 80° C., at least 90° C., or at least100° C. For example, the temperature can be in a range of 80° C. to 150°C., 90° C. to 150° C., or 90° C. to 125° C. for at least 30 min, atleast 60 min, or at least 120 min.

If desired, at least parts of the surface of the 3-dim article being inan absorbent stage can be coloured. Colouring can be effected by usingcolouring solutions.

A suitable colouring solution typically contains a solvent and certainions.

The solvent is able to dissolve the ion(s) contained in the treatmentsolution. If desired, mixtures of different solvents can be used.

Suitable solvents include water, alcohols (especially low-boilingalcohols, e.g. with a boiling point below about 100° C.) and ketones.Specific examples of solvents which can be used for dissolving thecations of the non-colouring agent include water, methanol, ethanol,iso-propanol, n-propanol, butanol, acetone, and mixtures thereof.

The solvent is typically present in an amount from 50 to 99.9 wt.-% orfrom 60 to 99 wt.-% or from 75 to 95 wt.-%, wt.-% with respect to thewhole colouring solution.

The colouring solution has typically an adequate viscosity so that asufficient amount of solution can not only be applied to the surface ofthe porous zirconia article but also is able to migrate into the poresof the zirconia article.

Adjusting the viscosity to a value as indicated above can be beneficialin that the solution can be more accurately applied to particularsections or regions of the porous dental zirconia article obtained afterpresintering the printed ceramic article.

If the viscosity of the colouring solution is too high, the colouringsolution might not be able to sufficiently enter the pores of thezirconia material. On the other hand, if the viscosity of the colouringsolution is too low, the colouring solution might migrate into the porestoo rapidly and might diffuse into the whole article.

According to one embodiment, the colouring solution comprises coloringions selected from ions of Fe, Mn, Er, Pr, Tb, V, Cr, Co, Mo andmixtures thereof. These ions were found to be particularly useful foruse in the dental or orthodontic field.

The solution may also contain phase stabilizers including ions of Y, Ce,Mg, Ca, rare earth elements and mixtures thereof. The addition of phasestabilizers may further facilitate the stabilization of a certaincrystalline phase (e.g. cubic or tetragonal phase) of the zirconiacomponents present in the ceramic article.

The solution may also contain on or more complexing agent(s). Thecomplexing agent(s) may support the penetration of the coloringsolution.

The colouring solution can be applied to the surface of the 3-dimarticle with the help of application devices including brushes, sponges,(hollow) needles, pens, mixing appliances and combinations thereof.

However, the colouring ions can also be added to the sol for use in theadditive manufacturing process as described in the present text. In thiscase, the obtained article is already coloured.

A sintering step is finally carried out to obtain a ceramic articlehaving a density of at least 99 or at least 99.5 or at least 99.9% ofthe theoretical density.

Sintering of the 3-dim article is typically carried out under theflowing conditions:

-   -   temperature: from 1150 to 1500° C. or from 1200 to 1400° C. or        from 1250 to 1350° C. or from 1200 to 1400° or from above 1300        to 1400° C. or above 1320° C. to 1400° C. or above 1340° C. or        above 1350° C.;    -   atmosphere: air or inert gas (e.g. nitrogen, argon);    -   pressure: ambient pressure (e.g. 1013 mbar);    -   duration: until a density of about 99 to about 100% of the final        density of the material has been reached.

Properties of the sintered ceramic article are described in the presenttext further down below.

The preparation of the printing sol used in the additive manufacturingprocess typically starts with the preparation of a starting sol.

A precursor solution is prepared by combining a zirconium salt (e.g.acetate) solution and a solvent (e.g. water). A phase stabilizing agent(e.g. yttrium acetate) is added and dissolved in the precursor solution.The resulting composition is pumped e.g. through a hydrothermal reactor.

Suitable hydrothermal reactors are described e.g. in U.S. Pat. No.5,453,262 (Dawson et al.) and U.S. Pat. No. 5,652,192 (Matson et al.).

The content of tetragonal and/or cubic phase of the zirconiacrystallites can be adjusted by varying the amount of phase stabilizingcomponents added during the production method.

Phase stabilizing components which can be used include Ce, Mg, Ca, Y,rare earth elements and combinations thereof.

Although any of a variety of known methods can be used to provide thezirconia-based particles, preferably they are prepared usinghydrothermal technology.

In one exemplary embodiment, the zirconia-based sols are prepared byhydrothermal treatment of aqueous metal salt (e.g., a zirconium salt, anyttrium salt, and an optional lanthanide element salt or aluminum salt)solutions, suspensions or a combination of them.

The aqueous metal salts, which are selected to be soluble in water, aretypically dissolved in the aqueous medium. The aqueous medium can bewater or a mixture of water with other water soluble or water misciblematerials. In addition, the aqueous metal salts and other water solubleor water miscible materials which may be present are typically selectedto be removable during subsequent processing steps and to benon-corrosive.

At least a majority of the dissolved salts in the feedstock are usuallycarboxylate salts rather than halide salts, oxyhalide salts, nitratesalts, or oxynitrate salts. Although not wanting to be bound by theory,it is believed that halide and nitrate anions in the feedstock tend toresult in the formation of zirconia-based particles that arepredominately of a monoclinic phase rather than the more desirabletetragonal or cubic phases. Further, carboxylates and/or acids thereoftend to be more compatible with an organic matrix material compared tohalides and nitrates. Although any carboxylate anion can be used, thecarboxylate anion often has no greater than 4 carbon atoms (e.g.,formate, acetate, propionate, butyrate, or a combination thereof). Thedissolved salts are often acetate salts. The feedstock can furtherinclude, for example, the corresponding carboxylic acid of thecarboxylate anion. For example, feedstocks prepared from acetate saltsoften contain acetic acid.

One exemplary zirconium salt is zirconium acetate salt, represented by aformula such as ZrO_(((4-n)/2)) ^(n+)(CH3COO⁻)_(n), where n is in therange from 1 to 2. The zirconium ion may be present in a variety ofstructures depending, for example, on the pH of the feedstock. Suitableaqueous solutions of zirconium acetate are commercially available, forexample, from Magnesium Elektron, Inc., Flemington, N.J., that contain,for example, up to 17 wt.-% zirconium, up to 18 wt.-% zirconium, up to20 wt.-% zirconium, up to 22 wt.-%, up to 24 wt.-%, up to 26 wt.-%, andup to 28 wt.-% zirconium, based on the total weight of the solution.

Similarly, exemplary yttrium salts, and aluminum salts often have acarboxylate anion, and are commercially available. Because these saltsare typically used at much lower concentration levels than the zirconiumsalt, however, salts other than carboxylate salts (e.g., acetate salts)may also be useful (e.g., nitrate salts).

The total amount of the various salts dissolved in the feedstock can bereadily determined based on the total percent solids selected for thefeedstock. The relative amounts of the various salts can be calculatedto provide the selected composition for the zirconia-based particles.

Typically, the pH of the feedstock is acidic. For example, the pH isusually less than 6, less than 5, or even less than 4 (in someembodiments, in a range from 3 to 4).

The liquid phase of the feedstock is typically predominantly water(i.e., the liquid phase is an aqueous based medium). Preferably, thewater is deionized to minimize the introduction of alkali metal ions,alkaline earth ions, or both into the feedstock. Optionally,water-miscible organic co-solvents are included in the liquid phase inamounts, for example, up 20 wt.-%, based on the weight of the liquidphase. Suitable co-solvents include 1-methoxy-2-propanol, ethanol,iso-propanol, ethylene glycol, N,N-dimethylacetamide, and N-methylpyrrolidone.

When subjected to hydrothermal treatment, the various dissolved salts inthe feedstock undergo hydrolysis and condensation reactions to form thezirconia-based particles. These reactions are often accompanied with therelease of an acidic by-product. That is, the by-product is often one ormore carboxylic acids corresponding to the zirconium carboxylate saltplus any other carboxylate salt in the feedstock. For example, if thesalts are acetate salts, acetic acid is formed as a by-product of thehydrothermal reaction.

Any suitable hydrothermal reactor can be used for the preparation of thezirconia-based particles. The reactor can be a batch or continuousreactor. The heating times are typically shorter and the temperaturesare typically higher in a continuous hydrothermal reactor compared to abatch hydrothermal reactor. The time of the hydrothermal treatments canbe varied depending, for example, on the type of reactor, thetemperature of the reactor, and the concentration of the feedstock. Thepressure in the reactor can be autogeneous (i.e., the vapour pressure ofwater at the temperature of the reactor), can be hydraulic (i.e., thepressure caused by the pumping of a fluid against a restriction), or canresult from the addition of an inert gas such as nitrogen or argon.Suitable batch hydrothermal reactors are available, for example, fromParr Instruments Co., Moline, Ill.

In some embodiments, the feedstock is passed through a continuoushydrothermal reactor. As used herein, the term “continuous” withreference to the hydrothermal reactor system means that the feedstock iscontinuously introduced and an effluent is continuously removed from theheated zone. The introduction of feedstock and the removal of theeffluent typically occur at different locations of the reactor. Thecontinuous introduction and removal can be constant or pulsed.

The dimensions of the tubular reactor can be varied and, in conjunctionwith the flow rate of the feedstock, can be selected to provide suitableresidence times for the reactants within the tubular reactor. Anysuitable length tubular reactor can be used provided that the residencetime and temperature are sufficient to convert the zirconium in thefeedstock to zirconia-based particles. The tubular reactor often has alength of at least 0.5 meter (in some embodiments, at least 1 m, 2 m, 5m, 10 m, 15 m, 20 m, 30 m, 40 m, or even at least 50 m). The length ofthe tubular reactor in some embodiments is less than 500 m (in someembodiments, less than 400 m, 300 m, 200 m, 100 m, 80 m, 60 m, 40 m, oreven less than 20 m).

Tubular reactors with a relatively small inner diameter are sometimespreferred. For example, tubular reactors having an inner diameter nogreater than about 3 cm are often used because of the fast rate ofheating of the feedstock that can be achieved with these reactors. Also,the temperature gradient across the tubular reactor is less for reactorswith a smaller inner diameter compared to those with a larger innerdiameter. The larger the inner diameter of the tubular reactor, the morethis reactor resembles a batch reactor. However, if the inner diameterof the tubular reactor is too small, there is an increased likelihood ofthe reactor becoming plugged or partially plugged during operationresulting from deposition of material on the walls of the reactor. Theinner diameter of the tubular reactor is often at least 0.1 cm (in someembodiments, at least 0.15 cm, 0.2 cm, 0.3 cm, 0.4 cm, 0.5 cm, or evenat least 0.6 cm). In some embodiments, the diameter of the tubularreactor is no greater than 3 cm (in some embodiments, no greater than2.5 cm, 2 cm, 1.5 cm, or even greater than 1 cm; in some embodiments, ina range from 0.1 to 2.5 cm, 0.2 cm to 2.5 cm, 0.3 cm to 2 cm, 0.3 cm to1.5 cm, or even 0.3 cm to 1 cm).

In a continuous hydrothermal reactor, the temperature and the residencetime are typically selected in conjunction with the tubular reactordimensions to convert at least 90 mole percent of the zirconium in thefeedstock to zirconia-based particles using a single hydrothermaltreatment. That is, at least 90 mole percent of the dissolved zirconiumin the feedstock is converted to zirconia-based particles within asingle pass through the continuous hydrothermal reactor system.

Alternatively, for example, a multiple step hydrothermal process can beused. For example, the feedstock can be subjected to a firsthydrothermal treatment to form a zirconium-containing intermediate and aby-product such as a carboxylic acid. A second feedstock can be formedby removing at least a portion of the by-product of the firsthydrothermal treatment from the zirconium-containing intermediate. Thesecond feedstock can then be subjected to a second hydrothermaltreatment to form a sol containing the zirconia-based particles. Furtherdetails on this process are described, for example, in U.S. Pat. No.7,241,437 (Davidson et al.).

If a two-step hydrothermal process is used, the percent conversion ofthe zirconium-containing intermediate is typically in a range from 40 to75 mol-%. The conditions used in the first hydrothermal treatment can beadjusted to provide conversion within this range. Any suitable methodcan be used to remove at least part of the by-product of the firsthydrothermal treatment. For example, carboxylic acids such as aceticacid can be removed by a variety of methods such as vaporization,dialysis, ion exchange, precipitation, and filtration.

When referring to a continuous hydrothermal reactor, the term “residencetime” means the average length of time that the feedstock is within theheated portion of the continuous hydrothermal reactor system.

Any suitable flow rate of the feedstock through the tubular reactor canbe used as long as the residence time is sufficiently long to convertthe dissolved zirconium salt to zirconia-based particles. That is, theflow rate is often selected based on the residence time needed toconvert the zirconium in the feedstock to zirconia-based particles.Higher flow rates are desirable for increasing throughput and forminimizing the deposition of materials on the walls of the tubularreactor. A higher flow rate can often be used when the length of thereactor is increased or when both the length and diameter of the reactorare increased. The flow through the tubular reactor can be eitherlaminar or turbulent.

In some exemplary continuous hydrothermal reactors, the reactortemperature is in the range from 170° C. to 275° C., 170° C. to 250° C.,170° C. to 225° C., 180° C. to 225° C., 190° C. to 225° C., 200° C. to225° C., or even 200° C. to 220° C. If the temperature is greater thanabout 275° C., the pressure may be unacceptably high for somehydrothermal reactors systems. However, if the temperature is less thanabout 170° C., the conversion of the zirconium slat in the feedstock tozirconia-based particles may be less than 90 wt.-% using typicalresidence times.

The effluent of the hydrothermal treatment (i.e., the product of thehydrothermal treatment) is a zirconia-based sol and can be referred toas the “sol effluent”. The sol effluent is a dispersion or suspension ofthe zirconia-based particles in the aqueous-based medium. The soleffluent contains at least 3 wt.-% zirconia-based particles dispersed,suspended, or a combination thereof based on the weight of the sol. Insome embodiments, the sol effluent contains at least 5 wt.-%, at least 6wt.-%, at least 8 wt.-%, or at least 10 wt.-% zirconia-based particlesbased on the weight of the sol. The wt.-% zirconia-based particles canbe up to 16 wt.-% or higher, up to 15 wt.-%, up to 12 wt.-%, or up to 10wt.-%.

The sol effluent usually contains non-associated zirconia-basedparticles. The sol effluent is typically clear or slightly cloudy. Incontrast, zirconia-based sols that contain agglomerated or aggregatedparticles usually tend to have a milky or cloudy appearance. The soleffluent often has a high optical transmission due to the small size andnon-associated form of the primary zirconia particles in the sol. Highoptical transmission of the sol effluent can be desirable in thepreparation of transparent or translucent sintered articles. As usedherein, “optical transmission” refers to the amount of light that passesthrough a sample (e.g., a sol effluent or printing sol) divided by thetotal amount of light incident upon the sample. The percent opticaltransmission may be calculated using the equation100(I/Io)where I is the light intensity Pa*ssing though the sample and Io is thelight intensity incident on the sample.

The optical transmission through the sol effluent is often related tothe optical transmission through the printing sol (reaction mixture usedto form the gel body). Good transmission helps ensure that adequatecuring occurs during the formation of the gel body.

The optical transmission may be determined using an ultraviolet/visiblespectrophotometer set, for example, at a wavelength of 420 nm or 600 nmwith a 1 cm path length. The optical transmission is a function of theamount of zirconia in a sol. For sol effluents containing about 1 wt.-%zirconia, the optical transmission is typically at least 70 percent, atleast 80 percent, at least 85 percent, or at least 90 percent at either420 nm or 600 nm. For sol effluents containing about 10 wt.-% zirconia,the optical transmission is typically at least 20 percent, at least 25percent, at least 30 percent, at least 40 percent, at least 50 percent,or at least 70 percent at either 420 nm or 600 nm.

The zirconia-based particles in the sol effluent are crystalline and canbe cubic, tetragonal, monoclinic, or a combination thereof. Because thecubic and tetragonal phases are difficult to differentiate using x-raydiffraction techniques, these two phases are typically combined forquantitative purposes and are referred to as the “cubic/tetragonal”phase. The percent cubic/tetragonal phase can be determined, forexample, by measuring the peak area of the x-ray diffraction peaks foreach phase and using the following equation:% C/T=100(C/T)÷(C/T+M)

In this equation, “C/T” refers to the area of the diffraction peak forthe cubic/tetragonal phase, “M” refers to the area of the diffractionpeak for the monoclinic phase, and “% C/T” refers to the wt.-%cubic/tetragonal crystalline phase. The details of the x-ray diffractionmeasurements are described further in the Example section below.

Typically, at least 50 wt.-% of the zirconia-based particles in the soleffluent have a cubic structure, a tetragonal structure, or acombination thereof. A greater content of the cubic/tetragonal phase isusually desired. The amount of cubic/tetragonal phase is often at least60 wt.-%, at least 70 wt.-%, at least 75 wt.-%, at least 80 wt.-%, atleast 85 wt.-%, at least 90 wt.-%, or at least 95 wt.-% based on a totalweight of all crystalline phases present in the zirconia-basedparticles.

Different sols having different yttria contents can be mixed in order toadjust the ratio of cubic to tetragonal phase content of the zirconiacrystallites contained therein, if desired.

The zirconia-based particles within the sol effluent are crystalline andhave an average primary particle size no greater than 50 nm, no greaterthan 45 nm, no greater than 40 nm, no greater than 30 nm, no greaterthan 20 nm, no greater than 15 nm, or no greater than 10 nm. Thezirconia-based particles typically have an average primary particle sizethat is at least 1 nm, at least 2 nm, at least 3 nm, at least 4 nm, orat least 5 nm. In some embodiments, the average primary particle size isin a range of 5 to 50 nm, 5 to 45 nm, 2 to 40 nm, 5 to 40 nm, 2 to 25nm, 5 to 25 nm, 2 to 20 nm, 5 to 20 nm, 2 to 15 nm, 5 to 15 nm, or 2 to10 nm. The average primary particle size, which refers to thenon-associated particle size of the zirconia particles, can bedetermined by x-ray diffraction as described in the Example section.

The particles in the sol effluent are typically non-associated and theaverage particle size is the same as the primary particle size. Theextent of association between the primary particles can be determinedfrom the volume-average particle size. The volume-average particle sizecan be measured using Photon Correlation Spectroscopy as described inmore detail in the Examples section below. Briefly, the volumedistribution (percentage of the total volume corresponding to a givensize range) of the particles is measured. The volume of a particle isproportional to the third power of the diameter. The volume-average sizeis the size of a particle that corresponds to the mean of the volumedistribution. If the zirconia-based particles are associated, thevolume-average particle size provides a measure of the size of theaggregate and/or agglomerate of primary particles. If the particles ofzirconia are non-associated, the volume-average particle size provides ameasure of the size of the primary particles. The zirconia-basedparticles typically have a volume-average size of up to 100 nm. Forexample, the volume-average size can be up to 90 nm, up to 80 nm, up to75 nm, up to 70 nm, up to 60 nm, up to 50 nm, up to 40 nm, up to 30 nm,up to 25 nm, up to 20 nm, or up to 15 nm, or even up to 10 nm.

A quantitative measure of the degree of association between the primaryparticles in the sol effluent is the dispersion index. As used hereinthe “dispersion index” is defined as the volume-average particle sizedivided by the primary particle size. The primary particle size (e.g.,the weighted average crystallite size) is determined using x-raydiffraction techniques and the volume-average particle size isdetermined using Photon Correlation Spectroscopy. As the associationbetween primary particles decreases, the dispersion index approaches avalue of 1 but can be somewhat higher or lower. The zirconia-basedparticles typically have a dispersion index in a range of from 1 to 7.For example, the dispersion index is often in a range 1 to 5, 1 to 4, 1to 3, 1 to 2.5, or even 1 to 2.

Photon Correlation Spectroscopy also can be used to calculate theZ-average primary particle size. The Z-average size is calculated fromthe fluctuations in the intensity of scattered light using a cumulativeanalysis and is proportional to the sixth power of the particlediameter. The volume-average size will typically be a smaller value thanthe Z-average size. The zirconia-based particles tend to have aZ-average size that is up to 100 nm. For example, the Z-average size canbe up to 90 nm, up to 80 nm, up to 70 nm, up to 60 nm, up to 50 nm, upto 40 nm, up to 35 nm, up to 30 nm, up to 20 nm, or even up to 15 nm.

Depending on how the zirconia-based particles are prepared, theparticles may contain at least some organic material in addition to theinorganic oxides. For example, if the particles are prepared using ahydrothermal approach, there may be some organic material attached tothe surface of the zirconia-based particles. Although not wanting to bebound by theory, it is believed that organic material originates fromthe carboxylate species (anion, acid, or both) included in the feedstockor formed as a by-product of the hydrolysis and condensation reactions(i.e., organic material is often adsorbed on the surface of thezirconia-based particles). For example, in some embodiments, thezirconia-based particles contain up to 15 wt.-%, up to 12 wt.-%, up to10 wt.-%, up to 8 wt.-%, or even up to 6 wt.-% organic material based ona total weight of the zirconia-based particles.

The starting sol typically contains at least 2 wt.-% zirconia-basedparticles dispersed, suspended, or a combination thereof in an aqueousmedium. In some embodiments, the zirconia-based particles can contain(a) 0 to 10 mol-% of a lanthanide element oxide, based on total moles ofinorganic oxide in the zirconia-based particles, and (b) 0 to 30 mol-%yttrium oxide, based on total moles of inorganic oxide in thezirconia-based particles. The zirconia-based particles are crystallineand have an average primary particle size no greater than 50 nm or nogreater than 45 nm. In some embodiments, cerium oxide, magnesium oxide,ytterbium oxide, and/or calcium oxide may be used with or in place ofthe yttria.

Depending on the intended use of the final sintered articles, otherinorganic oxides can be included in the zirconia-based particles inaddition to zirconium oxide.

Thus, according to one embodiment, the sol described in the present textmay comprise one or more inorganic colouring agent(s). The nature andstructure of the inorganic colouring agent(s) is not particularlylimited, either unless the desired result cannot be achieved.

In preferred embodiments the metal ion is not a free salt, but rather isincorporated into the zirconia-based particles.

Up to 30 mol-%, up to 25 mol-%, up to 20 mol-%, up to 10 mol-%, up to 5mol-%, up to 2 mol-%, or up to 1 mol-% of the zirconia-based particlescan be Y₂O₃, La₂O₃, Al₂O₃, CeO₂, Pr₂O₃, Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃,Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Fe₂O₃, MnO₂, Co₂O₃,Cr₂O₃, NiO, CuO, V₂O₃, Bi₂O₃, Ga₂O₃, Lu₂O₃, HfO₂, or mixtures thereof.Inorganic oxide such as Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Ga₂O₃,Er₂O₃, Pr₂O₃, Eu₂O₃, Dy₂O₃, Sm₂O₃, V₂O₃, or W₂O₃ may be added may beadded, for example, to alter the colour of the ceramic article to beproduced.

If the sol is to be used for producing dental or orthodontic articles,the following inorganic colouring agent (s) were found to be useful:ions of Mn, Fe, Cu, Pr, Nd, Sm, Eu, Tb, Dy, Er, Bi and mixtures thereof,preferably ions of Er, Tb, Mn, Bi, Nd or Fe, Pr, Co, Cr or V, Cu, Eu,Sm, Dy, with Er, Tb, Mn, Bi, Nd being sometimes particularly preferred.

If present, the inorganic colouring agent(s) is present in the followingamounts:

Lower amount: at least 0.001 or at least 0.005 or at least 0.01 wt.-%;

Upper amount: at most 0.02 or at most 0.05 or at most 0.5 wt.-%;

Range: from 0.001 to 0.5 or from 0.005 to 0.05 wt.-%;

calculated on the weight of the colouring ion being present in thecolouring agent and with respect to the weight of the sol. The startingsol is typically concentrated.

To obtain a more concentrated sol, at least a portion of theaqueous-based medium is removed from the zirconia-based sol. Any knownmeans for removing the aqueous-based medium can be used. Thisaqueous-based medium contains water and often contains dissolvedcarboxylic acids and/or anions thereof that are present in the feedstockor that are by-products of the reactions that occur within thehydrothermal reactor. As used herein, the term “carboxylic acids and/oranions thereof” refers to carboxylic acids, carboxylate anions of thesecarboxylic acids, or mixtures thereof. The removal of at least a portionof these dissolved carboxylic acids and/or anions thereof from thezirconia-based sol may be desirable in some embodiments. Thezirconia-based sol can be subjected, for example, to at least one ofvaporization, drying, ion exchange, solvent exchange, diafiltration, ordialysis, for example, for concentrating, removal of impurities or tocompatibilize with other components present in the sol.

According to one embodiment, the zirconia based sol can be subjected todialysis or diafiltration.

Dialysis and diafiltration both tend to remove at least a portion of thedissolved carboxylic acids and/or anions thereof. For dialysis, a sampleof the effluent can be positioned within a membrane bag that is closedand then placed within a water bath. The carboxylic acid and/orcarboxylate anions diffuse out of the sample within the membrane bag.That is, these species will diffuse out of the effluent through themembrane bag into the water bath to equalize the concentration withinthe membrane bag to the concentration in the water bath. The water inthe bath is typically replaced several times to lower the concentrationof species within the bag. A membrane bag is typically selected thatallows diffusion of the carboxylic acids and/or anions thereof but doesnot allow diffusion of the zirconia-based particles out of the membranebag.

For diafiltration, a permeable membrane is used to filter the sample.The zirconia particles can be retained by the filter if the pore size ofthe filter is appropriately chosen. The dissolved carboxylic acidsand/or anions thereof pass through the filter. Any liquid that passesthrough the filter is replaced with fresh water. In a discontinuousdiafiltration process, the sample is often diluted to a pre-determinedvolume and then concentrated back to the original volume byultrafiltration. The dilution and concentration steps are repeated oneor more times until the carboxylic acid and/or anions thereof areremoved or lowered to an acceptable concentration level. In a continuousdiafiltration process, which is often referred to as a constant volumediafiltration process, fresh water is added at the same rate that liquidis removed through filtration. The dissolved carboxylic acid and/oranions thereof are in the liquid that is removed.

While the majority of the yttrium and lanthanum, if present, areincorporated into the crystalline zirconia particles there is a fractionof these metals that can be removed during the diafiltration or dialysisprocess. The actual composition of a sol after diafiltration may bedifferent than that before dialysis.

The content of the crystalline nano-sized zirconia particles in theconcentrated starting sol is typically in a range from 20 to 70 wt.-%.

In some embodiments, the concentrated zirconia based sol can besubjected to a solvent exchange process.

An organic solvent having a higher boiling point than water can be addedto the effluent. Examples of organic solvents that are suitable for usein a solvent exchange method include 1-methoxy-2-propanol, N-methylpyrrolidone or diethylene glycol ethyl ether. The water then can beremoved by a method such as distillation, rotary evaporation, or ovendrying. Depending on the conditions used for removing the water, atleast a portion of the dissolved carboxylic acid and/or anion thereofcan also be removed. Other organic matrix material can be added to thetreated effluent (i.e., other organic matrix material can be added tothe zirconia-based particle suspended in the organic solvent used in thesolvent exchange process).

A zirconia based sol comprises zirconia-based particles dispersed and/orsuspended (i.e., dispersed, suspended, or a combination thereof) in anaqueous/organic matrix.

The sol to be used in the additive manufacturing process described inthe present text (printing sol) is obtained by adding further componentsto the starting sol.

This can include addition of surface modifiers, addition of radiationcurable monomer(s) or oligomer(s), photoinitiators, inhibitors, organicdyes, solvents and a mixture or combination thereof. The concentrationsand a composition can be further adjusted through diafiltration,distillation or comparable processes, if desired.

In certain embodiments the printing sol to be used in the additivemanufacturing process can be characterized by at least one or more,sometimes all of the following parameters:

-   a) being translucent in a wave length range from 420 to 600 nm;-   b) showing a transmission of at least 5% at 420 nm determined for a    path length of 10 mm;-   c) substantially free of associated nano-sized zirconia particles;-   d) being acidic, i.e. having a pH in the range of 1 to 6 or 2 to 5    or 2 to 4 if brought in contact with water;-   e) viscosity: less than 200 or less than 180 or less than 150 or    less than 100 or less than 50 or less than 20 mPa*s at 23° C.

It was found that using a translucent printing sol can be beneficial forimproving the printing results or detail resolution of the surface ofthe ceramic article. In contrast to e.g. a slips or slurries,translucent printing sols as described in the present text show lessscattering of light, which is used for polymerizing the radiationcurable components contained in the sol.

The increased translucency allows for a more shallow cure gradient,which may also allow for a more uniform cure across the entire structureto be obtained, as lower energy doses are required to cure through atranslucent material.

Many of the compositions and slurries suggested as construction materialin an additive manufacturing process described in the prior art are nottranslucent but rather opaque due to the larger size of the dispersedparticles.

The printing sol described in the present text allows transmission ofultraviolet/visible radiation.

The percent transmission of the printing sol containing 40 wt.-%zirconia-based particles is typically at least 5% when measured at 420nm in a 1 cm sample cell (i.e., the spectrophotometer has a 1 cm pathlength). In some examples, the percent transmission under these sameconditions is at least 7%, at least 10% and can be up to 20% or higher,up to 15%, or up to 12%. The percent transmission of a printing solcomposition containing 40 wt.-% zirconia-based particles is typically atleast 20% when measured at 600 nm in a 1-centimeter sample cell. In someexamples, the percent transmission under these same conditions is atleast 30%, at least 40% and can be up to 80% or higher, up to 70%, or upto 60%. The printing sol is translucent and not opaque. In someembodiments, the cured green body gel compositions are translucent aswell.

The transmission of the ultraviolet/visible radiation should besufficiently high to form a gel composition layer that adheres to thepreviously built gel composition layer in a manner that minimizes overbuild for sufficient realization of the digital file input shape in thegel composition. The “slice thickness” is often slightly smaller thanthe depth of cure to allow for layer adhesion. Minimizing the differencebetween “slice thickness” and cure depth can be advantageous forenhanced resolution.

Further, filled radiation curable compositions, e.g. slurries or slips,typically result in a reduction in green body strength compared tonon-filled curable compositions. Smaller non-associated particlestypically provide more surface area for the two phases to interact in anSLA process. This may result in a more robust green body or green bodygel.

In addition, using nanoscale particles theoretically hold the potentialfor printing a nanoscale layer. This would not be possible with largerparticles. For example, a 25-μm layer thickness setting with a slurrythat has 50 μm particles doesn't allow for fine realization of thefidelity offered with a small layer thickness. This includes not onlythe resolution of the material that makes up the part, but also thevoids and internal structures that can be realized within the 3D printedpart. This also includes the ability to realize ultrathin walls.

Similarly, sols with uniformly dispersed nanoscale particlestheoretically allow for smoother surface finishes to be achieved ascompared to those with larger particles.

Finally, smaller nanoparticles provide opportunity for a more uniformand higher density final sintered part, resulting in greater mechanicaland optical performance of the part, important for several applications.

Using an acidic sol is also often beneficial. It was found that anacidic sol containing nano-zirconia particles is more stable than aneutral or basic sol. The risk of formation of agglomerated oraggregated particles contained therein is reduced. However, the solshould not be too acidic. If the sol is too acidic, the risk of theformation of agglomerated or aggregated particles is sometimesincreased.

A printing sol having a viscosity in the above range is beneficial e.g.as it can easily be processed through thin nozzles and tubes. There isno need for heating either the nozzles or tubes of the manufacturingunit or the construction material itself.

Typically, a reaction mixture containing filler particles (e.g. a slurryor slip) is significantly more viscous than an unfilled reactionmixture. This typically results in longer build times as the materialneeds more time to flow into position, requirements of additionalmechanical actuation to move the material in place, and difficulty inremoving the material from the surface of the part (especiallyconsidering a lower green body strength) when shaping of the article isfinished.

Reducing the viscosity of the reaction mixture can provide improvedperformance and efficiency.

Low viscosity sols also allow for simple removal of excess materialsfrom internal channels or deep slots, such as in the case of a narrowwire slot on an orthodontic bracket. Higher viscosity sols combined witha green body of lower mechanical strength make the likelihood of fullyrealizing the small resolution of channels and dips rather low.

A sol having a viscosity in the ranges described in the present text canbe beneficial, e.g. as it can easily flow back over the cured area forproducing the next layer.

It can also facilitate better layer adhesion by being more able tointimately wet out the previous cured layer.

The printing sol typically has a viscosity that is sufficiently low sothat it can allow for the formation of small, complex features. In manyembodiments, the printing sol has a viscosity that is Newtonian ornearly Newtonian like. That is, the viscosity is independent of shearrate or has only a slight dependence on shear rate.

The viscosity can vary depending on the percent solids of the reactionmixture, the size of the zirconia-based particles, the composition ofthe solvent medium, the presence or absence of optionalnon-polymerizable surface modification agents, and the composition ofthe polymerizable material.

The combination of low viscosity and small particle size of thezirconia-based particles advantageously allows the printing sol to befiltered before polymerization. The reaction mixture is often filteredprior to stereolithographic processing. Filtering can be beneficial forremoval of debris and impurities that can negatively impact theproperties of the gel composition and properties of the sintered articlesuch as optical transmission and strength. Suitable filters often have apore size of 0.22 μm, 0.45 μm, 1 μm, 2 μm, or 5 μm. Traditional ceramicprinting compositions cannot be easily filtered due to particle sizeand/or viscosity.

In some embodiments, the viscosity of the printing sol is at least 2mPa*s, at least 5 mPa*s, at least 10 mPa*s, at least 25 mPa*s, at least50 mPa*s, at least 100 mPa*s, or at least 150 mPa*s. The viscosity canbe up to 200 mPa*s, up to 100 mPa*s, up to 50 mPa*s, up to 30 mPa*s, orup to 10 mPa*s. For example, the viscosity can be in a range of 2 to 200mPa*s, 2 to 100 mPa*s, 2 to 50 mPa*s, 2 to 30 mPa*s, 2 to 20 mPa*s, or 2to 10 mPa*s.

Processing the printing sol described in the present text in astereolithography-based process also allows for easy and efficientremoval of excess of non-polymerized material from the surface of theprinted parts.

The printing sol for use in the additive manufacturing process describedin the present text comprises one or more solvents.

The nature and structure of the solvent is not particularly limitedunless the desired result cannot be achieved.

In certain embodiments the solvent(s) can be characterized by at leastone or more, sometimes all of the following parameters:

-   -   Boiling point: above 70 or above 100 or above 120 or above 150°        C.;    -   Molecular weight: from 25 to 300 g/mol or from 30 to 250 g/mol        or from 40 to 200 g/mol or from 50 to 175 g/mol;    -   Viscosity: from 0.1 to 50 or from 0.2 to 10 or from 0.3 to 5        mPa*s (23° C.);    -   miscible with water;    -   soluble in supercritical carbon dioxide or liquid carbon        dioxide.

Using a solvent with a boiling point above 70 or above 100 or above 150°C. can be beneficial for reducing the risk of evaporation of the solventduring the process.

Using a solvent with a molecular weight and/or viscosity in the aboverange can be beneficial as it helps adjusting the viscosity of the sol.The molecular weight size can also affect the diffusion constant and howeasily the solvent can be removed.

Using a mixture of different solvents can be beneficial as it allows toeven further adjust viscosity or post processing properties, e.g.removal of excess sol after printing.

The solvent should be able to dissolve or to disperse the othercomponents being present in the sol.

The solvent should generally also be unreactive towards other componentsbeing present in the composition.

The solvent should also be easily removable during the furtherprocessing steps needed for the realization of a ceramic article.

Further, the solvent should not interfere with or negatively influencethe polymerization of the radiation curable components being present inthe sol.

In this respect, using solvents not bearing polymerizable moieties canbe beneficial.

To enhance the dissolving capability or property of the solvent, thesolvent typically bears one or more polar moieties, including ether,alcohol or carboxy moieties.

According to one embodiment, the solvent is often a glycol orpolyglycol, mono-ether glycol or mono-ether polyglycol, di-ether glycolor di-ether polyglycol, ether ester glycol or ether ester polyglycol,carbonate, amide, or sulfoxide (e.g., dimethyl sulfoxide). The organicsolvents usually have one or more polar groups. The organic solvent doesnot have a polymerizable group; that is, the organic solvent is free ofa group that can undergo free radical polymerization. Further, nocomponent of the solvent medium has a polymerizable group that canundergo free radical polymerization.

Suitable glycols or polyglycols, mono-ether glycols or mono-etherpolyglycols, di-ether glycols or di-ether polyglycols, and ether esterglycols or ether ester polyglycols are often of the following Formula(I).R¹O—(R²O)_(n)—R¹  (I)In Formula (I), each R¹ independently is hydrogen, alkyl, aryl, or acyl.Suitable alkyl groups often have 1 to 10 carbon atoms, 1 to 6 carbonatoms, or 1 to 4 carbon atoms. Suitable aryl groups often have 6 to 10carbon atoms and are often phenyl or phenyl substituted with an alkylgroup having 1 to 4 carbon atoms. Suitable acyl groups are often offormula —(CO)R^(a) where R^(a) is an alkyl having 1 to 10 carbon atoms,1 to 6 carbon atoms, 1 to 4 carbon atoms, 2 carbon atoms, or 1 carbonatom. The acyl is often an acetyl group (i.e. —C(O)CH₃). In Formula (I),each R² is typically an alkylene group such as ethylene or propylene.The variable n is at least 1 and can be in a range of 1 to 10, 1 to 6, 1to 4, or 1 to 3.

Glycols or polyglycols of Formula (I) have two R¹ groups equal tohydrogen. Examples of glycols include, but are not limited to, ethyleneglycol, propylene glycol, diethylene glycol, dipropylene glycol,triethylene glycol, and tripropylene glycol.

Mono-ether glycols or mono-ether polyglycols of Formula (I) have a firstR¹ group equal to hydrogen and a second R¹ group equal to alkyl or aryl.Examples of mono-ether glycols or mono-ether polyglycols include, butare not limited to, ethylene glycol monohexyl ether, ethylene glycolmonophenyl ether, propylene glycol monobutyl ether, diethylene glycolmonomethyl ether, diethylene glycol monoethyl ether, diethylene glycolmonopropyl ether, diethylene glycol monobutyl ether, diethylene glycolmonohexyl ether, dipropylene glycol monomethyl ether, dipropylene glycolmonoethyl ether, dipropylene glycol monopropyl ether, triethylene glycolmonomethyl ether, triethylene glycol monoethyl ether, triethylene glycolmonobutyl ether, tripropylene glycol monomethyl ether, and tripropyleneglycol monobutyl ether.

Di-ether glycols or di-ether polyglycols of Formula (I) have two R1group equal to alkyl or aryl. Examples of di-ether glycols or di-etherpolyglycols include, but are not limited to, ethylene glycol dipropylether, ethylene glycol dibutyl ether, dipropylene glycol dibutyl ether,diethylene glycol dimethyl ether, diethylene glycol diethyl ether,triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether,and pentaethylene glycol dimethyl ether.

Ether ester glycols or ether ester polyglycols of Formula (I) have afirst R1 group equal to an alkyl or aryl and a second R1 group equal toan acyl. Examples of ether ester glycols or ether ester polyglycolsinclude, but are not limited to, ethylene glycol butyl ether acetate,diethylene glycol butyl ether acetate, and diethylene glycol ethyl etheracetate.

Other suitable organic solvents are carbonates of Formula (II).

In Formula (II), R³ is hydrogen or an alkyl such as an alkyl having 1 to4 carbon atoms, 1 to 3 carbon atoms, or 1 carbon atom. Examples includeethylene carbonate and propylene carbonate.

Yet other suitable organic solvents are amides of Formula (III).

In Formula (III), group R⁴ is hydrogen, alkyl, or combines with R⁵ toform a five-membered ring including the carbonyl attached to R⁴ and thenitrogen atom attached to R⁵. Group R⁵ is hydrogen, alkyl, or combineswith R⁴ to form a five-membered ring including the carbonyl attached toR⁴ and the nitrogen atom attached to R⁵. Group R⁶ is hydrogen or alkyl.Suitable alkyl groups for R⁴, R⁵, and R⁶ have 1 to 6 carbon atoms, 1 to4 carbon atoms, 1 to 3 carbon atoms, or 1 carbon atom. Examples of amideorganic solvents of Formula (III) include, but are not limited to,formamide, N,N-dimethylformamide, N,N-dimethylacetamide,N,N-diethylacetamide, N-methyl-2-pyrrolidone, and N-ethyl-2-pyrrolidone.

Specific examples of solvents which can be used include: mono alcohols(e.g. C2 to C8 alcohols, including primary, secondary and tertiaryalcohols), poly alcohols (e.g. ethylene glycol, propylene glycol,glycerine, diethylene glycol ethyl ether (Carbitol™),1-methoxy-2-propanol, N-methyl pyrrolidone acetonitrile, chlorobenzene,1,4-dioxane, ethyl acetate, methyl ethyl ketone, tetrahydrofuran,toluene, xylene and mixtures thereof.

The following solvents are sometimes preferred: ethanol,1-methoxy-2-propanol, N-methyl pyrrolidone, diethylene glycol ethylether, and mixtures thereof. In some situations, suitable solvents mayalso include low boiling alcohols (below 100° C.; like methanol,ethanol, propanol) and mixtures thereof or preferably the samesolvent(s) described above.

The solvent(s) is typically present in the following amounts:

Lower amount: at least 25 or at least 30 or at least 35 wt.-%;

Upper amount: at most 70 or at most 65 or at most 60 wt.-%;

Range: from 25 to 70 or from 30 to 65 or from 35 to 60 or from 35 to 55or from 35 to 50 wt.-%;

wt.-% with respect to the weight of the printing sol.

In one embodiment, the printing sol for use in the additivemanufacturing process described in the present text comprises nano-sizedzirconia particles.

The nature and structure of the nano-sized zirconia particles is notparticularly limited unless the desired result cannot be achieved.

The printing sol preferably comprises zirconia crystallites having acertain tetragonal phase content and a sol comprising zirconiacrystallites having a certain cubic phase content.

In certain embodiments the nano-sized zirconia particles(s) can becharacterized by at least one or more, sometimes all of the followingparameters or features:

-   -   Primary particle size XRD (diameter): from 2 to 50 or from 2 to        20 nm or from 2 to 15 or from 4 to 15 nm;    -   being essentially spherical, cuboid or a mixture of spherical        and cuboid;    -   being non-associated;    -   being crystalline;    -   not being coated with an inorganic coloring agent.        “Essentially spherical” means that the shape of the particles is        close to a sphere. It does not contain sharp edges, which may        result from a milling process.        The nano-sized zirconia particles contained in the sol can be        characterized by at least one or more or all of the following        features:    -   ZrO₂ content: from 70 to 100 mol-% or from 80 to 97 mol-%;    -   HfO₂ content: from 0 to 4.5 mol-% or from 0 to 3 mol-% or from        0.1 to 2.8 mol-%;    -   Stabilizer selected from Y₂O₃, CeO₂, MgO, CaO, La₂O₃ or a        combination thereof in an amount from 0 to 30 mol-% or from 1.5        to 16 mol-% or from 2 to 10 mol-% or 2 to 5 mol-%;    -   Al₂O₃ content: from 0 to 1 mol-% or from 0.005 to 0.5 mol-% or        from 0.01 to 0.2 mol-%.

According to one embodiment, the nano-sized zirconia particles arecharacterized as follows: ZrO₂ content: from 70 to 98.4 mol-%; HfO₂content: from 0.1 to 2.8 mol-%; Y₂O₃ content: from 1.5 to 20 mol-%.

The nano-sized zirconia particles can be obtained or are obtainable by aprocess comprising the steps of hydrothermal treatment of an aqueousmetal salt solution or suspension (e.g. zirconium salt, yttrium salt).Such a process is described in WO 2013/055432 (3M), the content of whichis herewith incorporated by reference.

With respect to wt.-%, the printing sol used to form the gel compositiontypically contains 20 to 70 wt.-% zirconia-based particles based on atotal weight of the printing sol. The amount of zirconia-based particlescan be at least 25 wt.-%, at least 30 wt.-%, at least 35 wt.-%, or atleast 40 wt.-% and can be up to 55 wt.-%, up to 50 wt.-%, or up to 45wt.-%. In some embodiments, the amount of the zirconia-based particlesare in a range of 25 to 55 wt.-%, 30 to 50 wt.-%, 30 to 45 wt.-%, 35 to50 wt.-%, 40 to 50 wt.-%, or 35 to 45 wt.-% based on the total weight ofthe printing sol.

If expressed in vol.-%, the amount of the zirconia-based particles is asfollows:

Lower amount: at least 2 or at least 4 or at least 5 vol.-%;

Upper amount: at most 25 or at most 18 at most 16 vol.-%;

Range: from 2 to 25 or from 4 to 18 vol.-% or from 5 to 16 vol.-%;

vol.-% with respect to the volume of the printing sol.

Compared to zirconia containing slurries or slips described in the priorart (e.g. U.S. Pat. No. 7,927,538 B2), the printing sol described in thepresent text contains the zirconia particles in only a comparably lowvolume content. This facilitates adjusting the sol to a desired lowviscosity, which may contribute to an easier processing. It may alsoallow the manufacturing of 3-dim articles in an enlarged state, whichcan be sintered to the desired dimension later, as described later inthe text.

The printing sol described in the present text comprises one or moreradiation curable monomers being part of or forming an organic matrix.

The radiation curable monomers being present in the printing sol can bedescribed as first, second, third, etc. monomer.

The nature and structure of the radiation curable monomer(s) is notparticularly limited unless the desired result cannot be achieved.

Upon polymerization, the radiation curable monomers form a network withthe homogeneously dispersed nano-sized zirconia particles.

According to one embodiment the printing sol described in the presenttext contains as a first monomer a polymerizable surface modificationagent.

A surface modification agent may help to improve compatibility of thezirconia particles contained in the sol with an organic matrix materialbeing present in the sol as well.

Surface modification agents may be represented by the formula A-B, wherethe A group is capable of attaching to the surface of a zirconia-basedparticle and the B group is radiation curable.

Group A can be attached to the surface of the zirconia-based particle byadsorption, formation of an ionic bond, formation of a covalent bond, ora combination thereof.

Examples for Group A include acidic moieties (like carboxylic acidgroups, phosphoric acid groups, sulfonic acid groups and anions thereof)and silanes.

Group B comprises a radiation curable moiety.

Examples for Group B include vinyl, in particular acryl or methacrylmoieties.

Suitable surface modifying agents comprise polymerizable carboxylicacids and/or anions thereof, polymerizable sulfonic acids and/or anionsthereof, polymerizable phosphoric acids and/or anions thereof, andpolymerizable silanes. Suitable surface modification agents are furtherdescribed, for example, in WO 2009/085926 (Kolb et al.), the disclosureof which is incorporated herein by reference.

An example of a radically polymerizable surface modifier is apolymerizable surface modification agent comprising an acidic moiety oranion thereof, e.g. a carboxylic acid group.

Exemplary acidic radically polymerizable surface modifiers includeacrylic acid, methacrylic acid, beta-carboxyethyl acrylate, andmono-2-(methacryloxyethyl)succinate.

Exemplary radically polymerizable surface modifiers can be reactionproducts of hydroxyl-containing polymerizable monomers with cyclicanhydrides such as succinic anhydride, maleic anhydride and phthalicanhydride. Exemplary polymerization hydroxyl-containing monomers includehydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropylacrylate, hydroxypropyl methacrylate, hydroxyl butyl acrylate, andhydroxybutyl methacrylate. Acryloxy and methacryloxy functionalpolyethylene oxide, and polypropylene oxide may also be used as thepolymerizable hydroxyl-containing monomers.

An exemplary radically polymerizable surface modifier for imparting bothpolar character and reactivity to the zirconia-containing nanoparticlesis mono(methacryloxypolyethyleneglycol) succinate.

Another example of a radically polymerizable surface modifier is apolymerizable silane.

Exemplary polymerizable silanes includemethacryloxyalkyltrialkoxysilanes, or acryloxyalkyltrialkoxysilanes(e.g., 3-methacryloxypropyltrimethoxysilane,3-acryloxypropyltrimethoxysilane, and3-(methacryloxy)propyltriethoxysilane; as3-(methacryloxy)propylmethyldimethoxysilane, and3-(acryloxypropyl)methyldimethoxysilane);methacryloxyalkyldialkylalkoxysilanes oracyrloxyalkyldialkylalkoxysilanes (e.g.,3-(methacryloxy)propyldimethylethoxysilane);mercaptoalkyltrialkoxylsilanes (e.g., 3-mercaptopropyltrimethoxysilane);aryltrialkoxysilanes (e.g., styrylethyltrimethoxysilane); vinylsilanes(e.g., vinylmethyldiacetoxysilane, vinyldimethylethoxysilane,vinylmethyldiethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane,vinyltriacetoxysilane, vinyltriisopropoxysilane, vinyltrimethoxysilane,and vinyltris(2-methoxyethoxy)silane).

A surface modification agent can be added to the zirconia-basedparticles using conventional techniques. The surface modification agentcan be added before or after any removal of at least a portion of thecarboxylic acids and/or anions thereof from the zirconia-based sol. Thesurface modification agent can be added before or after removal of thewater from the zirconia-based sol. The organic matrix can be addedbefore or after surface modification or simultaneously with surfacemodification. Various methods of adding the surface modification agentare further described, for example, in WO 2009/085926 (Kolb et al.), thedisclosure of which is incorporated herein by reference.

The surface modification reactions can occur at room temperature (e.g.,20° C. to 25° C.) or at an elevated temperature (e.g., up to 95° C.).When the surface modification agents are acids such as carboxylic acids,the zirconia-based particles typically can be surface-modified at roomtemperature. When the surface modification agents are silanes, thezirconia-based particles are typically surface modified at elevatedtemperatures.

The first monomer can function as a polymerizable surface modificationagent. Multiple first monomers can be used. The first monomer can be theonly kind of surface modification agent or can be combined with one ormore other non-polymerizable surface modification agents. In someembodiments, the amount of the first monomer is at least 20 wt.-% basedon a total weight of polymerizable material. For example, the amount ofthe first monomer is often at least 25 wt.-%, at least 30 wt.-%, atleast 35 wt.-%, or at least 40 wt.-%. The amount of the first monomercan be up to 100 wt.-%, up to 90 wt.-%, up to 80 wt.-%, up to 70 wt.-%,up to 60 wt.-%, or up to 50 wt.-%. Some printing sols contain 20 to 100wt.-%, 20 to 80 wt.-%, 20 to 60 wt.-%, 20 to 50 wt.-%, or 30 to 50 wt.-%of the first monomer based on a total weight of polymerizable material.

The first monomer (i.e. the polymerizable surface modification agent)can be the only monomer in the polymerizable material or it can becombined with one or more second monomers that are soluble in thesolvent medium.

According to one embodiment, the printing sol described in the presenttext comprises one or more second monomers comprising at least one ortwo radiation curable moieties. In particular the second monomerscomprising at least two radiation curable moieties may act ascrosslinker(s) during the gel-forming step.

Any suitable second monomer that does not have a surface modificationgroup can be used. The second monomer does not have a group beingcapable of attaching to the surface of a zirconia-based particle.

A successful build typically requires a certain level of green body gelstrength as well as shape resolution. A crosslinked approach oftenallows for greater green body gel strength to be realized at a lowerenergy dose since the polymerization creates a stronger network. In someexamples, higher energy doses have been applied to increase layeradhesion of non crosslinked systems. While an article is successfullybuilt, the higher energy often impacts the resolution of the finalarticle, causing overbuild to potentially occur, especially in the caseof highly translucent materials where the light and with it the curedepth can penetrate further into the material.

The presence of the monomer having a plurality of polymerizable groupstends to enhance the strength of the gel composition formed when theprinting sol is polymerized. Such gel compositions can be easier toprocess without cracking. The amount of the monomer with a plurality ofthe polymerizable groups can be used to adjust the flexibility and thestrength of the green body gel, and indirectly optimize the green bodygel resolution and final article resolution.

In the case where the light source is applied from below, it was foundthat applying e.g. crosslink chemistry may help to increase the strengthof the adhesion between layers so that when the build platform is raisedafter the cure step, the newly cured layer moves with the buildingshape, rather than being separated from the rest of the build and leftbehind on the transparent film, which would be considered a failedbuild.

A successful build could be defined as the scenario when the materialadheres better to the previously cured layers than the build tray filmto allow for a three-dimensional structure to be grown one layer at atime.

This performance could in theory be achieved by applying an increasedenergy dose (higher power, or longer light exposure) to provide astronger adhesion up to a certain point characteristic of the bulkmaterial. However, in a fairly transparent system where light absorbingadditives are not present a higher energy exposure will eventuallyprovide a depth of cure significantly greater than the ‘slice thickness’creating an over-cured situation where the resolution of the part issignificantly beyond that of the ‘slice thickness’.

Adding a radiation curable component comprising at least two radiationcurable moieties to the printing sol described in the present text mayfacilitate the optimization of resolution as well as green bodystrength.

In the case of transforming the green body into a fully dense ceramic,increased green body gel strength aids in the robustness of thepost-building procedures.

That is, the optional second monomer does not have a carboxylic acidgroup or a silyl group. The second monomers are often polar monomers(e.g., non-acidic polar monomers), monomers having a plurality ofpolymerizable groups, alkyl (meth)acrylates and mixtures thereof.

The overall composition of the polymerizable material is often selectedso that the polymerized material is soluble in the solvent medium.Homogeneity of the organic phase is often preferable to avoid phaseseparation of the organic component in the gel composition. This tendsto result in the formation of smaller and more homogeneous pores (poreswith a narrower size distribution) in the subsequently formed aerogel.Further, the overall composition of the polymerizable material can beselected to adjust compatibility with the solvent medium and to adjustthe strength, flexibility, and uniformity of the gel composition. Stillfurther, the overall composition of the polymerizable material can beselected to adjust the burnout characteristics of the organic materialprior to sintering.

In many embodiments, the second monomer includes a monomer having aplurality of polymerizable groups. The number of polymerizable groupscan be in a range of 2 to 6 or even higher. In many embodiments, thenumber of polymerizable groups is in a range of 2 to 5 or 2 to 4. Thepolymerizable groups are typically (meth)acryloyl groups.

Exemplary monomers with two (meth)acryloyl groups include 1,2-ethanedioldiacrylate, 1,3-propanediol diacrylate, 1,9-nonanediol diacrylate,1,12-dodecanediol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanedioldiacrylate, butylene glycol diacrylate, bisphenol A diacrylate,diethylene glycol diacrylate, triethylene glycol diacrylate,tetraethylene glycol diacrylate, tripropylene glycol diacrylate,polyethylene glycol diacrylate, polypropylene glycol diacrylate,polyethylene/polypropylene copolymer diacrylate, polybutadienedi(meth)acrylate, propoxylated glycerin tri(meth)acrylate, andneopentylglycol hydroxypivalate diacrylate modified caprolactone.

Exemplary monomers with three or four (meth)acryloyl groups include, butare not limited to, trimethylolpropane triacrylate (e.g., commerciallyavailable under the trade designation TMPTA-N from Cytec Industries,Inc. (Smyrna, Ga., USA) and under the trade designation SR-351 fromSartomer (Exton, Pa., USA)), pentaerythritol triacrylate (e.g.,commercially available under the trade designation SR-444 fromSartomer), ethoxylated (3) trimethylolpropane triacrylate (e.g.,commercially available under the trade designation SR-454 fromSartomer), ethoxylated (4) pentaerythritol tetraacrylate (e.g.,commercially available under the trade designation SR-494 fromSartomer), tris(2-hydroxyethylisocyanurate) triacrylate (e.g.,commercially available under the trade designation SR-368 fromSartomer), a mixture of pentaerythritol triacrylate and pentaerythritoltetraacrylate (e.g., commercially available from Cytec Industries, Inc.,under the trade designation PETIA with an approximately 1:1 ratio oftetraacrylate to triacrylate and under the trade designation PETA-K withan approximately 3:1 ratio of tetraacrylate to triacrylate),pentaerythritol tetraacrylate (e.g., commercially available under thetrade designation SR-295 from Sartomer), and di-trimethylolpropanetetraacrylate (e.g., commercially available under the trade designationSR-355 from Sartomer).

Exemplary monomers with five or six (meth)acryloyl groups include, butare not limited to, dipentaerythritol pentaacrylate (e.g., commerciallyavailable under the trade designation SR-399 from Sartomer) and ahexa-functional urethane acrylate (e.g., commercially available underthe trade designation CN975 from Sartomer).

Some printing sol compositions contain 0 to 80 wt.-% of a second monomerhaving a plurality of polymerizable groups based on a total weight ofthe polymerizable material. For example, the amount can be in a range of10 to 80 wt.-%, 20 to 80 wt.-%, 30 to 80 wt.-%, 40 to 80 wt.-%, 10 to 70wt.-%, 10 to 50 wt.-%, 10 to 40 wt.-%, or 10 to 30 wt.-%.

In some embodiments, the optional second monomer is a polar monomer. Asused herein, the term “polar monomer” refers to a monomer having a freeradical polymerizable group and a polar group. The polar group istypically non-acidic and often contains a hydroxyl group, a primaryamido group, a secondary amido group, a tertiary amido group, an aminogroup, or an ether group (i.e., a group containing at least onealkylene-oxy-alkylene group of formula —R—O—R— where each R is analkylene having 1 to 4 carbon atoms).

Suitable optional polar monomers having a hydroxyl group include, butare not limited to, hydroxyalkyl (meth)acrylates (e.g., 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl(meth)acrylate, and 4-hydroxybutyl (meth)acrylate), and hydroxyalkyl(meth)acrylamides (e.g., 2-hydroxyethyl (meth)acrylamide or3-hydroxypropyl (meth)acrylamide), ethoxylated hydroxyethyl(meth)acrylate (e.g., monomers commercially available from Sartomer(Exton, Pa., USA) under the trade designation CD570, CD571, and CD572),and aryloxy substituted hydroxyalkyl (meth)acrylates (e.g.,2-hydroxy-2-phenoxypropyl (meth)acrylate).

Exemplary polar monomers with a primary amido group include(meth)acrylamide. Exemplary polar monomers with secondary amido groupsinclude, but are not limited to, N-alkyl (meth)acrylamides such asN-methyl(meth)acrylamide, N-ethyl (meth)acrylamide, N-isopropyl(meth)acrylamide, N-tert-octyl (meth)acrylamide, and N-octyl(meth)acrylamide. Exemplary polar monomers with a tertiary amido groupinclude, but are not limited to, N-vinyl caprolactam,N-vinyl-2-pyrrolidone, (meth)acryloyl morpholine, and N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide,N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, andN,N-dibutyl(meth)acrylamide.

Polar monomers with an amino group include various N,N-dialkylaminoalkyl(meth)acrylates and N,N-dialkylaminoalkyl (meth)acrylamides. Examplesinclude, but are not limited to, N,N-dimethyl aminoethyl (meth)acrylate,N,N-dimethylaminoethyl (meth)acrylamide,N,N-dimethylaminopropyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylamide, N,N-diethylaminoethyl(meth)acrylate,N,N-diethylaminoethyl (meth)acrylamide,N,N-diethylaminopropyl(meth)acrylate, and N,N-diethylaminopropyl(meth)acrylamide.

Exemplary polar monomers with an ether group include, but are notlimited to, alkoxylated alkyl (meth)acrylates such as ethoxyethoxyethyl(meth)acrylate, 2-methoxyethyl (meth)acrylate, and 2-ethoxyethyl(meth)acrylate; and poly(alkylene oxide) (meth)acrylates such aspoly(ethylene oxide) (meth)acrylates, and poly(propylene oxide)(meth)acrylates. The poly(alkylene oxide) acrylates are often referredto as poly(alkylene glycol) (meth)acrylates. These monomers can have anysuitable end group such as a hydroxyl group or an alkoxy group. Forexample, when the end group is a methoxy group, the monomer can bereferred to as methoxy poly(ethylene glycol) (meth)acrylate.

Suitable alkyl (meth)acrylates that can be used as a second monomer canhave an alkyl group with a linear, branched, or cyclic structure.Examples of suitable alkyl (meth)acrylates include, but are not limitedto, methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate,isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl(meth)acrylate, n-pentyl (meth)acrylate, 2-methylbutyl (meth)acrylate,n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 4-methyl-2-pentyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-methylhexyl(meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, 2-octyl(meth)acrylate, isononyl (meth)acrylate, isoamyl (meth)acrylate,3,3,5-trimethylcyclohexyl (meth)acrylate, n-decyl (meth)acrylate,isodecyl (meth)acrylate, isobornyl (meth)acrylate, 2-propylheptyl(meth)acrylate, isotridecyl (meth)acrylate, isostearyl (meth)acrylate,octadecyl (meth)acrylate, 2-octyldecyl (meth)acrylate, dodecyl(meth)acrylate, lauryl (meth)acrylate, and heptadecanyl (meth)acrylate.

The amount of a second monomer that is a polar monomer and/or an alkyl(meth)acrylate monomer is often in a range of 0 to 40 wt.-%, 0 to 35wt.-%, 0 to 30 wt.-%, 5 to 40 wt.-%, or 10 to 40 wt.-% based on a totalweight of the polymerizable material.

The total amount of polymerizable material is often at least 2 wt.-%, atleast 3 wt.-%, at least 5 wt.-%, or at least 10 wt.-% based on the totalweight of the printing sol. The amount of polymerizable material can beup to 30 wt.-%, up to 25 wt.-%, up to 20 wt.-%, or up to 15 wt.-% basedon the total weight of the printing sol. For example, the amount ofpolymerizable material can be in a range of 2-30 wt.-%, 3-20 wt.-%, or5-15 wt.-% based on the total weight of the printing sol.

Overall, the polymerizable material typically contains 20 to 100 wt.-%first monomer and 0 to 80 wt.-% second monomer based on a total weightof polymerizable material. For example, polymerizable material includes30 to 100 wt.-% first monomer and 0 to 70 wt.-% second monomer, 30 to 90wt.-% first monomer and 10 to 70 wt.-% second monomer, 30 to 80 wt.-%first monomer and 20 to 70 wt.-% second monomer, 30 to 70 wt.-% firstmonomer and 30 to 70 wt.-% second monomer, 40 to 90 wt.-% first monomerand 10 to 60 wt.-% second monomer, 40 to 80 wt.-% first monomer and 20to 60 wt.-% second monomer, 50 to 90 wt.-% first monomer and 10 to 50wt.-% second monomer, or 60 to 90 wt.-% first monomer and 10 to 40 wt.-%second monomer.

In some applications, it can be advantageous to minimize the weightratio of polymerizable material to zirconia-based particles in thereaction mixture. This tends to reduce the amount of decompositionproducts of organic material that needs to be burned out prior toformation of the sintered article. The weight ratio of polymerizablematerial to zirconia-based particles is often at least 0.05, at least0.08, at least 0.09, at least 0.1, at least 0.11, or at least 0.12. Theweight ratio of polymerizable material to zirconia-based particles canbe up to 0.8, up to 0.6, up to 0.4, up to 0.3, up to 0.2, or up to 0.1.For example, the ratio can be in a range of 0.05 to 0.8, 0.05 to 0.6,0.05 to 0.4, 0.05 to 0.2, 0.05 to 0.1, 0.1 to 0.8, 0.1 to 0.4, or 0.1 to0.3.

In certain embodiments the second monomer(s) can be characterized by atleast one or more, sometimes all of the following parameters:

-   -   soluble in the solvent contained in the sol;    -   bearing at least one or two or three radiation curable moieties;    -   bearing radiation curable moieties selected from vinyl, acryl or        methacryl moieties;    -   molecular weight from 70 to 5,000 or from 70 to 1,000 g/mol or        from 100 to 500 g/mol.

Using radiation curable component(s) as described above having amolecular weight in the above range facilitates the provision of a solhaving the desired viscosity. Lower molecular weight components aretypically also better soluble than high molecular weight components.

If present, the second monomer is typically present in the followingamounts:

Lower amount: at least 0.5 or at least 1 or at least 3 wt.-%;

Upper amount: at most 5 or at most 10 or at most 24 wt.-%;

Range: from 0.5 to 24 or from 3 to 10 wt.-%;

wt.-% with respect to the weight of the printing sol.

Methods for adding a radiation curable monomer(s) to the nanoparticlesare known in the art. The radiation curable monomer(s) can be added, forexample, before or after any removal of at least a portion of thecarboxylic acids and/or anions thereof from the zirconia-containing sol.The radiation curable component can be added, for example, before orafter removal of the water from the zirconia-containing sol.

The radiation curable components can be added directly to thenanoparticles containing sol. The radiation curable monomer(s) that areadded are subsequently polymerized and/or crosslinked to form a gel.

The printing sol described in the present text comprises one or morephotoinitiator(s).

The nature and structure of the photoinitiator is not particularlylimited, either, unless the desired result cannot be achieved.

In certain embodiments the photoinitiator(s) can be characterized by atleast one or more, sometimes all of the following parameters:

-   -   Soluble in the solvent contained in the sol;    -   Radiation absorption: within a range from 200 to 500 or from 300        to 450 nm.

The photoinitiator should be able to start or initiate the curing orhardening reaction of the radiation curable component(s) being presentin the printing sol.

The following classes of photoinitiator(s) can be used: a) two-componentsystem where a radical is generated through abstraction of a hydrogenatom from a donor compound; b) one component system where two radicalsare generated by cleavage.

Examples of photoiniators according to type (a) typically contain amoiety selected from benzophenone, xanthone or quinone in combinationwith an aliphatic amine.

Examples of photoinitiators according to type (b) typically contain amoiety selected form benzoin ether, acetophenon, benzoyl oxime or acylphosphine.

Exemplary UV initiators include 1-hydroxycyclohexyl benzophenone(available, for example, under the trade designation “IRGACURE 184” fromCiba Specialty Chemicals Corp., Tarrytown, N.Y.),4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl) ketone (available, forexample, under the trade designation “IRGACURE 2529” from Ciba SpecialtyChemicals Corp.), 2-hydroxy-2-methylpropiophenone (available, forexample, under the trade designation “DAROCURE D111” from Ciba SpecialtyChemicals Corp. and bis(2,4,6-trimethylbenzoyl)-phenylposphineoxide(available, for example, under the trade designation “IRGACURE 819” fromCiba Specialty Chemicals Corp.).

The photoinitiator(s) is typically present in the following amounts:

Lower amount: at least 0.01 or at least 0.05 or at least 0.1 wt.-%;

Upper amount: at most 0.5 or at most 1 or at most 3 wt.-%;

Range: from 0.01 to 3 or from 0.01 to 1 wt.-% or 0.01 to 0.5 wt.-%;

wt.-% with respect to the weight of the printing sol.

The printing sol described in the present text may also comprise one ormore organic dye(s).

The nature and structure of the organic dye(s) is not particularlylimited unless the desired result cannot be achieved.

It was found that by adding an organic dye, the ability of thetranslucent sol described in the present text to absorb radiation can beenhanced.

In addition, it was found that adding an organic dye contributes tosuppress or to lower the transmission of scattered light in the sol.This often helps to improve the accuracy or detail resolution of thesurface of the ceramic article obtained from the additive manufacturingprocess.

In certain embodiments the organic dye(s) can be characterized by atleast one, more, of all of the following parameters:

-   -   soluble in the solvent contained in the sol;    -   radiation absorption: within a range from 200 to 500 or from 300        to 450 nm;    -   combustible without residues at temperature below 800° C.;    -   having a molecular weight in the range of 50 to 1,000 g/mol.

The organic dye does typically not contain elements or ions other thanalkaline metal ions (e.g. Li, Na, K), earth alkaline metal ions (e.g.Mg, Ca), C, N, O, H, S, P, halogen (F, Cl, Br). That is, the organic dyemolecule does typically not contain any heavy metal ions (e.g. metalions having an atomic mass above 40 or above 45.

Dyes which can be used include those containing a moiety selected formazo groups and/or aromatic (hetero) cycles or other systems withdelocalized pi-electrons. In particular dyes useful for colouring foodwere found to be useful.

Specific examples of dye(s) which can be used include riboflavin (E101),tartrazine (E102), isatin, azorubin (E122) and combinations thereof.

If present, the organic dye(s) is present in the following amounts:

Lower amount: at least 0.001 or at least 0.002 or at least 0.005 wt.-%;

Upper amount: at most 0.5 or at most 0.2 or at most 0.1 wt.-%;

Range: from 0.001 to 0.5 or from 0.002 to 0.2 or 0.005 to 0.1 wt.-%;

wt.-% with respect to the weight of the printing sol.

According to a further embodiment, the printing sol described in thepresent text comprises one or more inhibitor(s).

The nature and structure of the inhibitor(s) is not particularlylimited, either, unless the desired result cannot be achieved.

An inhibitor may extend the shelf life of the printing sol, help preventundesired side reactions, and adjust the polymerization process of theradiation curable component(s) present in the sol.

Adding one or more inhibitor(s) to the printing sol may further help toimproving the accuracy or detail resolution of the surface of theceramic article.

In particular it was found that adding inhibitor(s) to the printing soldescribed in the present text may help to enhance the resolution andaccuracy of the SLA process by attenuating or avoiding unwantedscattering effects, as well as increase the shelf life of the printingsol.

The inhibitor(s) should be soluble in the solvent contained in the sol.Inhibitors which can be used often comprise a phenol moiety.

Specific examples of inhibitor(s) which can be used include:p-methoxyphenol (MOP), hydroquinone monomethylether (MEHQ),2,6-di-tert-butyl-4-methyl-phenol (BHT; Ionol), phenothiazine,2,2,6,6-tetramethyl-piperidine-1-oxyl radical (TEMPO) and mixturesthereof.

If present, the inhibitor(s) is present in the following amounts:

Lower amount: at least 0.001 or at least 0.005 or at least 0.01 wt.-%;

Upper amount: at most 0.02 or at most 0.05 or at most 0.5 at most 1wt.-%;

Range: from 0.001 to 1 or from 0.005 to 0.05 wt.-%;

wt.-% with respect to the weight of the printing sol.

According to one embodiment, the printing sol described in the presenttext used as construction material in the additive manufacturing processis characterized as follows:

-   -   Solvent content: from 25 to 70 or from 40 to 65 wt.-%; the sol        having preferably a boiling point above 70° C. and being        selected from alcohols and glycol ethers;    -   Polymerizable material content: from 2 to 30 wt.-%, or from 3 to        20 wt.-% or from 5 to 15 wt.-%, the polymerizable material        comprising a first monomer having at least one radiation curable        moiety and an acidic or silyl moiety;    -   Photoinitiator content: from 0.01 to 3 or from 0.5 to 1.5 wt.-%;    -   Nano-sized crystalline zirconia particles content: from 20 to        70, or from 30 to 50 wt.-%;    -   Inhibitor content: from 0 to 0.25 or from 0.001 to 0.15 wt.-%;    -   Organic dye content: from 0 to 0.2 or from 0.01 to 0.1 wt.-%;    -   wt.-% with respect to the weight of the printing sol.

The sol described in the present text does typically not comprise one ormore or all of the following components:

-   -   wax(es) in an amount of more than 0.1 wt.-% or more than 0.01        wt.-%;    -   insoluble pigment(s) in an amount of more than 0.5 wt.-% or 0.1        or 0.01 wt.-%;    -   stabilizers comprising an N,N-dialkylamine group in an amount of        more than 0.5 wt.-% or 0.1 or 0.01 wt.-%;        wt.-% with respect to the weight of the printing sol.

Adding those components to the sol described in the present text mayresult in a composition, the processing of which may cause problems inadditive manufacturing processes.

In particular components being insoluble in the solvent like pigment(s)can sometimes cause problems, e.g. due to light scattering.

The sol described in the present text can be obtained as follows:

A starting sol containing nano-sized particles as described in thepresent text is provided.

To this starting sol the other components are added: the radiationcurable component(s), the photoinitiator(s), and optionally organicdye(s), optionally inhibitor(s) and optionally inorganic colouringagent(s), if desired.

The preparation of the sol is typically conducted under save lightconditions to avoid an undesired early polymerization.

The sol is typically stored in a suitable device like a vessel, abottle, cartridge or container before use.

Sols which were found to be suitable as well are described e.g. in WO2013/055432 (3M) relating to aerogels, calcined articles and crystallinearticles comprising zirconia and a methods of making the same. U.S. Pat.No. 7,429,422 (Davidson et al.) also describes methods of makingzirconia-based sols, which can be used. Further sols which can be usedare described in U.S. application No. 62/127,569 (3M) filed Mar. 3,2015. The above references are herewith incorporated by reference.

The invention is also directed to a ceramic article obtained orobtainable by a process as described in the present text. According toone embodiment, the ceramic article is a dental or orthodontic ceramicarticle.

In certain embodiments the ceramic article can be characterized by atleast one or more, sometimes all of the following parameters:

-   -   density: at least about 98.5 (in some embodiments, 99, 99.5,        99.9, or even at least 99.99) percent of theoretical density;    -   Vickers hardness: from 450 MPa to 2,200 MPa, or from 500 MPa to        1,800 MPa·HV(2);    -   phase content: tetragonal phase: from 0 to 100 wt.-% or from 10        to 100 wt.-%; cubic phase: from 0 to 100 wt.-% or from 50 to 90        wt.-%;    -   flexural strength: from 450 MPa to 2,200 MPa, or from 500 MPa to        2,000 MPa; according to ISO 6872;    -   translucency: more than 30% determined on a polished sample        having a thickness of 1 mm;    -   at least one of the x, y, z dimensions being at least 0.25 or at        least 1 mm.

The average grain size is often in a range of 75 nm to 400 nm or in arange of 100 nm to 400 nm. The grain size is typically no greater than400 nm, no greater than 350 nm, no greater than 300 nm, no greater than250 nm, no greater than 200 nm, or no greater than 150 nm. It is assumedthat this grain size contributes to the high strength of the sinteredarticles.

The ceramic article obtainable or obtained according to the processdescribed in the present text often shows a laminated structure, if cutin longitudinal direction. Longitudinal direction means the directionaccording to which the additive manufacturing process took place.

In certain embodiments the ceramic article can be characterized by atleast one or more, sometimes all of the following features:

-   -   ZrO₂ content: from 70 to 100 mol-% or from 80 to 97 mol-%;    -   HfO₂ content: from 0 to 4.5 mol-% or from 0 to 3 mol-% or from        0.1 to 2.8 mol-%;    -   Stabilizer selected from Y₂O₃, CeO₂, MgO, CaO, La₂O₃ or a        combination thereof in an amount from 0 to 30 mol-% or from 1.5        to 20 mol-% or from 2 to 10 mol-% or 2 to 5 mol-%;    -   Al₂O₃ content: from 0 to 1 mol-% or from 0.005 to 0.5 mol-% or        from 0.01 to 0.2 mol-%;    -   optionally comprising oxides of elements selected from Er, Tb,        Mn, Bi, Nd, Fe, Pr, Co, Cr, V, Cu, Eu, Sm, Dy, Tb, preferably        Fe, Mn, Er, Pr, Tb and combinations thereof.

The ceramic article may have different shapes.

According to one embodiment, the ceramic article has the shape of adental ceramic article, in particular the shape of a dental restoration(including the shape of a dental crown, bridge, inlay, onlay, veneer,implant as described above) or orthodontic bracket.

All components used in the dental composition of the invention should besufficiently biocompatible, that is, the composition should not producea toxic, injurious, or immunological response in living tissue.

The production of the ceramic article does typically not require theapplication of a hot isostatic pressing step (HIP).

According to one embodiment, the process for producing a ceramic articleas described in the present text does not comprise either or all of thefollowing steps.

-   -   heating the construction material during the processing step to        a temperature above 70° C.;    -   applying pressure during the sintering process.

The complete disclosures of the patents, patent documents, andpublications cited herein are incorporated by reference in theirentirety as if each were individually incorporated. Variousmodifications and alterations to this invention will become apparent tothose skilled in the art without departing from the scope and spirit ofthis invention. The above specification, examples and data provide adescription of the manufacture and use of the compositions and methodsof the invention. The invention is not limited to the embodimentsdisclosed herein. One skilled in the art will appreciate that manyalternative embodiments of the invention can be made without departingfrom the spirit and scope of thereof.

The following examples are given to illustrate, but not limit, the scopeof this invention. Unless otherwise indicated, all parts and percentagesare by weight.

The invention described in the present text is also directed to thefollowing embodiments:

Embodiment OD1

A process for producing a ceramic article, the process comprising thesteps of:

-   -   providing a printing sol, the printing sol comprising nano-sized        zirconia particles, radiation curable components and a photo        initiator system,    -   processing the printing sol as construction material in an        additive manufacturing process to obtain a 3-dim article being        in a gel state, the 3-dim article having a Volume A,    -   applying a supercritical drying step to the 3-dim article being        in a gel state,    -   applying a heat treatment step to obtain a sintered 3-dim        ceramic article having the Volume F.

Embodiment OD2

The process according to preceding embodiment, Volume A of the 3-dimarticle in a gel state being more than 200% of the Volume F of the 3-dimarticle in its sintered state.

The printing sol described in the Embodiments OD1 to OD8 can be producedand processed in the same manner as described for the other printingsols of the present text.

The invention also relates to a process of producing a 3-dim article,the process comprising the step of processing a printing sol asdescribed in the present text by using or applying an additivemanufacturing technique as described in the present text.

The invention is also directed to a zirconia gel body obtainable orobtained according to the process described in the present text, thezirconia gel body being preferably a zirconia aerogel body or zirconiaxerogel body.

Examples

Unless otherwise indicated, all parts and percentages are on a weightbasis, all water is de-ionized water, and all molecular weights areweight average molecular weight. Moreover, unless otherwise indicatedall experiments were conducted at ambient conditions (23° C.; 1013mbar).

The following examples are given to illustrate, but not limit, the scopeof this invention.

Materials

Material or abbreviation Description MEEAA 2-(2-(2-Methoxyethoxy)Ethoxy) Acetic Acid Zirconium An aqueous solution of zirconium acetatecontaining acetate nominally 16.3 weight percent zirconium obtained fromMagnesium Elektron, Inc., Flemington, NJ, USA. The aqueous solution wasexposed to an ion exchange resin (obtained under the trade designation“AMBERLYTE IR 120” from Rohm and Haas Company, Philadelphia, PA, USA)before use (oxide content 21.85 wt. %). Lanthanum Lanthanum (III) oxide(99% rare earth oxides) Oxide Yttrium acetate Yttrium (III) acetatetetrahydrate (oxide content 33.4 wt. %). Lanthanum Lathanum (III)acetate hydrate (oxide content Acetate 45.5 wt. %) DI water De-ionizedwater. HEMA 2-Hydroxyethyl methacrylate Irgacure ™ UV/Visiblephotoinitiator from BASF 819 HEAA N-(2-Hydroxyethyl) acrylamide MOPMethoxyphenolMethodsMethod for Determining Total Pore Volume, Average Connected PoreDiameter and BET Surface Area

Total pore volume and average pore diameter were analyzed with the useof N₂ sorption Isotherms and BET surface area analysis. Samples ofaround 0.3-0.5 grams were cut if necessary from larger samples in orderto be inserted in to the straight tubes. All samples were degassed formore than 1 day at 100° C. before analysis. The samples were thenanalyzed by adsorption and desorption of N₂ gas with a QuantachromeAutosorb IQ (Quantachrome Instruments, Florida, USA) in a 12 mm cellwith no bulb and without a rod. Absorption data points are collectedfrom 0.0003 to 0.995 P/P0 and desorption points collected from 0.995 to0.05 P/P0. The analysis has been duplicated (or triplicated ifrepeatability was not ideal), and the averaged results reported. Thespecific surface area S was calculated by the BET method (Details 10regarding calculation see Autosorb-1 Operating Manual Ver. 1.51 IV.Theory and Discussion; Quantachrome Instruments, Inc.). The total porevolume V_(liq) is derived from the amount of vapor adsorbed at arelative pressure close to unity (p/p0 closest to 1), by assuming thatthe pores are then filled with liquid adsorbate (Details regardingcalculation see Autosorb-1 Operating Manual Ver. 1.51 IV. Theory andDiscussion; Quantachrome Instruments, Inc.). The average pore diameter(d) is calculated from the surface area (S) and the total pore volumeV_(liq):d=4V_(liq)/S. Total pore volume and average pore diameter arereported as determined by Nonlocal Density Functional Theory method.

Method for Measuring Archimedes Density

The measurements were made on a precision balance (identified as “AE160” from Mettler Instrument Corp., Hightstown, N.J., USA) using adensity determination kit (identified as “ME 33360” from MettlerInstrument Corp., Hightstown, N.J.). In this procedure the sample wasfirst weighed in air (A), then immersed in water (B) and weighed. Thewater was distilled and deionized. One drop of a wetting agent (obtainedunder trade designation “TERGITOL-TMN-6” from Dow Chemical Co., Danbury,Conn., USA) was added to 250 ml of water. The density was calculatedusing the formula p=(A/(A−B))ρ0, where ρ0 is the density of water. Therelative density can be calculated by reference to the theoreticaldensity (ρt) of the material, ρrel=(ρ/ρt)100.

Method for Measuring Flexural Strength of Ceramic Article

The flexural strength was determined according to ISO 6872 (2008). Thetest piece is printed and processed according to the invention in theshape of a flex bar with approximate dimensions of 1 mm×4 mm×12 mm. Bothlarge faces of the flex bar were polished down to a surface finish from15 micron grade diamond lapping film (668X Diamond Lapping Film PSA, 3M,St. Paul, Minn.) on a Beta, Grinder-Polisher (Buehler, Lake Bluff,Ill.), operating at 100 rpm and lubricated with water. Each of the 4edges along the length of the flex bar were chamfered, meaning to createa bevel on the edges of the specimens along, to a 45 degree angle. A3-point beam bend test configuration with a span of 10.0 mm wasemployed. The crosshead test speed was 1 mm/min. An Instron 5954 testframe (Instron Corporation, Canton, Mass.) was utilized. A minimum of 5samples were measured to determine the average strength.

Method for Measuring Translucency of Ceramic Article

If desired, the translucency of the ceramic articles can be evaluatedwith the following procedure. The test piece is printed and processed asdescribed in the present text in the shape of a disc with approximatedimensions of 1±0.03 mm thick×13 mm diameter. The parallel large facesof the disc were polished down to a surface finish from 15 micron gradediamond lapping film (668X Diamond Lapping Film PSA, 3M, St. Paul,Minn.) on a Beta, Grinder-Polisher (Buehler, Lake Bluff, Ill.),operating at 100 rpm and lubricated with water. The polished sample wasmeasured with a spectrophotometer (X-Rite Color i7, Grand Rapids, USA)in reflectance mode. Translucency (T) is determined according toT=1−RB/RW where RB=reflectance through a ceramic disc on a blacksubstrate and RW=reflectance through the same disc on a white substrate.Higher values of translucency are indicative of greater transmission oflight, and less opacity. A minimum of 5 samples were measured todetermine the average translucency.

Method for Crystalline Structure and Size (XRD Analysis)

If desired, the crystalline structure of the ceramic articles can beevaluated with the following procedure. Dried zirconia samples areground by hand using an agate mortar and pestle. A liberal amount of thesample is applied by spatula to a glass microscope slide on which asection of double-sided adhesive tape has been adhered. The sample ispressed into the adhesive on the tape by forcing the sample against theadhesive with the spatula blade. Excess sample is removed by scrapingthe sample area with the edge of the spatula blade, leaving a thin layerof particles adhered to the adhesive. Loosely adhered materialsremaining after the scraping are removed by forcefully tapping themicroscope slide against a hard surface. In a similar manner, corundum(Linde 1.0 μm alumina polishing powder, Lot Number C062, Union Carbide,Indianapolis, Ind.) is prepared and used to calibrate the X-raydiffractometer for instrumental broadening.

X-ray diffraction scans are obtained using a Philips verticaldiffractometer having a reflection geometry, copper Kα radiation, and aproportional detector registry of the scattered radiation. Thediffractometer is fitted with variable incident beam slits, fixeddiffracted beam slits, and a graphite diffracted beam monochromator. Thesurvey scan is recorded from 25 to 55 degrees two theta (2θ) using astep size of 0.04 degrees and a dwell time of 8 seconds. X-ray generatorsettings of 45 kV and 35 mA are used. Data for the corundum standard iscollected on three separate areas of several individual corundum mounts.Likewise, data is collected on three separate areas of the thin layersample mount.

The observed diffraction peaks are identified by comparison to referencediffraction patterns contained within the International Center forDiffraction Data (ICDD) powder diffraction database (sets 1-47, ICDD,Newton Square, Pa., USA). The diffraction peaks for the samples areattributed to either cubic/tetragonal (C/T) or monoclinic (M) forms ofzirconia. For zirconia-based particles, the (111) peak for the cubicphase and (101) peak for the tetragonal phase could not be separated sothese phases are reported together. The amounts of each zirconia formare evaluated on a relative basis, and the form of zirconia having themost intense diffraction peak is assigned the relative intensity valueof 100. The strongest line of the remaining crystalline zirconia form isscaled relative to the most intense line and given a value between 1 and100.

Peak widths for the observed diffraction maxima due to corundum aremeasured by profile fitting. The relationship between mean corundum peakwidths and corundum peak position (2θ) is determined by fitting apolynomial to these data to produce a continuous function used toevaluate the instrumental breadth at any peak position within thecorundum testing range. Peak widths for the observed diffraction maximadue to zirconia are measured by profile fitting the observed diffractionpeaks. The following peak widths are evaluated depending on the zirconiaphase found to be present:

-   -   Cubic/Tetragonal (C/T): (1 1 1)    -   Monoclinic (M): (−1 1 1), and (1 1 1)        A Pearson VII peak shape model with Kα1 and Kα2 wavelength        components and linear background model are used for all        measurements. Widths are calculated as the peak full width at        half maximum (FWHM) having units of degrees. The profile fitting        is accomplished by use of the capabilities of the JADE        diffraction software suite. Sample peak widths are evaluated for        the three separate data collections obtained for the same thin        layer sample mount.

Sample peaks are corrected for instrumental broadening by interpolationof instrumental breadth values from corundum instrument calibration andcorrected peak widths converted to units of radians. The Scherrerequation is used to calculate the primary crystal size.Crystallite Size (D)=Kλ/β(cos θ)

In the Scherrer equation, K is the form factor (here 0.9), λ is thewavelength (1.540598 Å), β is the calculated peak width after correctionfor instrumental broadening (in radians), and θ equals half the peakposition (scattering angle). β is equal to [calculated peakFWHM−instrumental breadth] (converted to radians) where FWHM is fullwidth at half maximum. The cubic/tetragonal (C/T) mean crystallite sizeis measured as the average of three measurements using (1 1 1) peak.That is,C/T mean crystallite size=[D(1 1 1)_(area1) +D(1 1 1)_(area2) +D(1 11)_(area3)]/3.The monoclinic (M) crystallite size is measured as the average of threemeasurements using the

-   -   (−1 1 1) peak and three measurements using the (1 1 1) peak.        M mean crystallite size=[D(−1 1 1)area 1+D(−1 1 1)area 2+D(−1 1        1)area 3+D(1 1 1)area 1+D(1 1 1)area 2+D(1 1 1)area 3]/6        The weighted average of the cubic/tetragonal (C/T) and        monoclinic phases (M) is calculated.        Weighted average=[(% C/T)(C/T size)+(% M)(M size)]/100        In this equation, % C/T equals the percent crystallinity        contributed by the cubic and tetragonal crystallite content of        the ZrO₂ particles; C/T size equals the size of the cubic and        tetragonal crystallites; % M equals the percent crystallinity        contributed by the monoclinic crystallite content of the ZrO₂        particles; and M size equals the size of the monoclinic        crystallites.        Method for Photon Correlation Spectroscopy (PCS)

If desired, particle size measurements are made using a light scatteringparticle sizer equipped with a red laser having a 633 nm wavelength oflight (obtained under the trade designation “ZETA SIZER—Nano Series,Model ZEN3600” from Malvern Instruments Inc., Westborough, Mass.). Eachsample is analyzed in a one-centimeter square polystyrene samplecuvette. The sample cuvette is filled with about 1 gram of deionizedwater, and then a few drops (about 0.1 gram) of the zirconia-based solare added. The composition (e.g., sample) within each sample cuvette ismixed by drawing the composition into a clean pipette and dischargingthe composition back into the sample cuvette several times. The samplecuvette is then placed in the instrument and equilibrated at 25° C. Theinstrument parameters are set as follows: dispersant refractive index1.330, dispersant viscosity 0.8872 MPa-second, material refractive index2.10, and material absorption value 0.10 units. The automaticsize-measurement procedure is then run. The instrument automaticallyadjusted the laser-beam position and attenuator setting to obtain thebest measurement of particle size.

The light scattering particle-sizer illuminates the sample with a laserand analyzed the intensity fluctuations of the light scattered from theparticles at an angle of 173 degrees. The method of Photon CorrelationSpectroscopy (PCS) is used by the instrument to calculate the particlesize. PCS uses the fluctuating light intensity to measure Brownianmotion of the particles in the liquid. The particle size is thencalculated to be the diameter of sphere that moves at the measuredspeed.

The intensity of the light scattered by the particle is proportional tothe sixth power of the particle diameter. The Z-average size or cumulantmean is a mean calculated from the intensity distribution and thecalculation is based on assumptions that the particles are mono-modal,mono-disperse, and spherical. Related functions calculated from thefluctuating light intensity are the Intensity Distribution and its mean.The mean of the Intensity Distribution is calculated based on theassumption that the particles are spherical. Both the Z-average size andthe Intensity Distribution mean are more sensitive to larger particlesthan smaller ones.

The Volume Distribution gives the percentage of the total volume ofparticles corresponding to particles in a given size range. Thevolume-average size is the size of a particle that corresponds to themean of the Volume Distribution. Since the volume of a particle isproportional to the third power of the diameter, this distribution isless sensitive to larger particles than the Z-average size. Thus, thevolume-average will typically be a smaller value than the Z-averagesize.

Method for Determining Dispersion Index (DI)

The dispersion index is equal to the volume-average size measured usingPhoton Correlation Spectroscopy divided by the weighted averagecrystallite size measured by XRD.

Method for Determining Polydispersity Index (PI)

The polydispersity index is a measure of the breadth of the particlesize distribution and is calculated along with the Z-average size in thecumulants analysis of the intensity distribution using PhotonCorrelation Spectroscopy. For values of the polydispersity index of 0.1and below, the breadth of the distribution is considered narrow. Forvalues above 0.5, the breadth of the distribution is considered broadand it is unwise to rely on the Z-average size to fully characterize theparticle size. Instead, one should characterize the particles using adistribution analysis such as the intensity or volume distribution. Thecalculations for the Z-average size and polydispersity index are definedin the ISO 13321:1996 E (“Particle size analysis—Photon correlationspectroscopy”, International Organization for Standardization, Geneva,Switzerland).

Method for Determining pH-Value

If desired, the measurement of the pH-value can be achieved by meansknown by the person skilled in art. E.g. an instrument like Metrohm™ 826can be used.

Method for Measuring Wt.-% Solids

The wt.-% solids can be determined by drying a sample weighing 3-6 gramsat 120° C. for 30 min. The percent solids can be calculated from theweight of the wet sample (i.e., weight before drying, weight_(wet)) andthe weight of the dry sample (i.e., weight after drying, weight_(dry))using the following equation: wt-% solids=100(weight_(dry))/weight_(wet).

Method for Measuring Oxide Content

The oxide content of a sol sample can be determined by measuring thepercent solids content as described in the “Method for Measuring Wt.-%Solids” then measuring the oxide content of those solids as described inthis section.

The oxide content of a solid is measured via thermal gravimetricanalysis (obtained under the trade designation “TGA Q500” from TAInstruments, New Castle, Del., USA). The solids (about 50 mg) are loadedinto the TGA and the temperature was taken to 900° C. The oxide contentof the solid is equal to the residual weight after heating to 900° C.

Method for Determining Vol.-% Oxide

The vol.-% oxide in a sol can be determined by first using a volumetricflask to measure the mass of a known volume of sol, which gives the soldensity ρ_(s) in grams/ml. Then, using the wt.-% oxide (measured asdescribed above in “Method for Measuring Oxide Content”), the vol.-%oxide is calculated as: vol.-% oxide=(ρ_(s)*wt.-% oxide)/(oxidedensity), where a value of 6.05 grams/ml is used for the oxide density.

Method for Determining Viscosity

If desired, the viscosity can be measured using a Brookfield Cone andPlate Viscometer (Model Number DV II available from BrookfieldEngineering Laboratories, Middleboro, Mass., USA). The measurements areobtained using spindle CPE-42. The instrument is calibrated withBrookfield Fluid I which gives a measured viscosity of 5.12 mPa*s (cp)at 192 l/sec (50 RPM). The compositions are placed in the measurementchamber. Measurements are made at 3-4 different RPM (revolutions perminute). The measured viscosity is not significantly affected by theshear rate. The shear rate is calculated as 3.84 multiplied by the RPM.Viscosity values, if reported are for the minimum shear rate where thetorque is in range.

Method for Determining Light Transmission (% T)

If desired, the light transmission can be measured using a Perkin ElmerLambda 35 UV/VIS Spectrometer (available from Perkin Elmer Inc.,Waltham, Mass., USA). The transmission is measured in a 10-mm quartzcuvette, with a water-filled 10-mm quartz cuvette as the reference. Theaqueous ZrO₂ sols can be measured at 1 and 10 weight % ZrO₂.

Vickers Hardness

If desired, the Vickers hardness can be determined according to ISO843-4 with the following modifications:

The surface of the samples are ground using silicon carbide grindingpaper (P400 and P1200). The test forces are adjusted to the hardnesslevel of samples. Used test forces were between 0.2 kg and 2 kg and wereapplied for 15 s each indentation. A minimum of 10 indentations ismeasured to determine the average Vickers hardness. The tests can beconducted with a hardness tester Leco M-400-G (Leco Instrumente GmbH).

A Starting Sol Preparation

Sol compositions are reported in mole percent inorganic oxide. Thefollowing hydrothermal reactor was used for preparing the sol. Thehydrothermal reactor was prepared from 15 meters of stainless steelbraided smooth tube hose (0.64 cm inside diameter, 0.17 cm thick wall;obtained under the trade designation “DuPont T62 CHEMFLUOR PTFE” fromSaint-Gobain Performance Plastics, Beaverton, Mich.). This tube wasimmersed in a bath of peanut oil heated to the desired temperature.Following the reactor tube, a coil of an additional 3 m of the samestainless steel braided smooth tube hose plus 3 m of 0.64 cmstainless-steel tubing with a diameter of 0.64 cm and wall thickness of0.089 cm that was immersed in an ice-water bath to cool the material anda backpressure regulator valve was used to maintain an exit pressure of3.45 MPa.

A precursor solution was prepared by combining 2,000 g zirconium acetatesolution with 1,889.92 g deionized water. 112.2 g yttrium acetate wereadded while mixing until fully dissolved. The solids content of theresulting solution was measured gravimetrically (120° C./hr. forced airoven) to be 19.31 wt.-%. 65.28 g deionized water were added to adjustthe final concentration to 19 wt.-%. The resulting solution was pumpedat a rate of 11.48 ml/min. through the hydrothermal reactor. Thetemperature was 225° C. and the average residence time was 42 min. Aclear and stable zirconia sol was obtained.

The resulting sol was concentrated (35-45 wt.-% solids) viaultrafiltration using a membrane cartridge (obtained under the tradedesignation “M21S-100-01P” from Spectrum Laboratories Inc., RanchoDominguez, Calif.). As medium for the filtration process ethanol wasused. With that, next to increasing the solid content of the sol, apartial exchange of water with ethanol was achieved.

The size of the obtained zirconia particles was in the range of 15 to 25nm according to light scattering measurement (Z-average).

B Sol Preparation

A diethylene glycol ethyl ether based sol was produced by adding theappropriate amount of diethylene glycol ethyl ether (adjusted to theintended final concentration of zirconia in the sol, e.g. 50 wt.-%) tothe ethanolic sol and afterwards ethanol and water were removed bygentle heating under reduced pressure. Additionally, 2,2,2-MEEAA(2-[2-(2-methoxyethoxy)ethoxy]acetic acid) was added for adjusting theviscosity.

C Printing Sol Preparation

4.3 wt.-% acrylic acid, 2.2 wt.-% hydroxyethylacrylamid (HEAA), 0.08wt.-% photoinitiator (Irgacure™ 819), 0.02 wt.-% dye (Isatin:1H-indole-2,3-dione) and 0.02 wt.-% inhibitor (MOP) were added to thesol.

TABLE 1 Final composition of printing sol wt.-% Oxides (96:4 ZrO2:Y2O3mole) 50 Acetic acid 2.4 Diethylene glycol ethyl ether 38.48 Water 1Acrylic acid 4.3 Hydroxyethylacrylamid 2.2 Irgacure ™ 819 0.082-[2-(2-methoxyethoxy)ethoxy]acetic acid 1.5 Isatin 0.02 MOP 0.02C Processing of Printing SolC1 Manufacturing of a Flat Cuboid

The printing sol was processes as follows:

The sol was poured into the working tray of a commercially availableSLA/DLP printer (Rapidshape S50; Heimsheim, Germany).

The following conditions were applied:

-   -   curing light wavelength: 405 nm light;    -   exposure time: 2 sec;    -   layer thickness: 50 μm;    -   printing protocol: using the standard parameter set for material        GP101 (Software: Netfabb Professional for Rapidshape 5.2 64        bit).

The obtained sample was post-cured for 60 sec in a UV light furnace(Heraeus, Germany) and stored in air (FIG. 1A).

The sample was dried applying the following supercritical conditions:

-   -   liquid: carbon dioxide;    -   pressure: 200 bar;    -   temperature: 100° C.;    -   duration: 40 h.

The sample obtained after conducting the supercritical drying step isshown in FIG. 1B,

The sample was heated to 1035° C. for 72 h to burnout the remainingorganic components. The sample obtained after this burnout andpre-sintering step is shown in FIG. 1C.

Finally the sample was sintered at 1300° C. for 2 h (FIG. 1D).

The sample did not show any cracks. Further, the sample had a smoothsurface.

C2 Manufacturing of a Cube

According to the same process a cuboid (having a different geometry) wasmanufactured.

FIG. 2 shows that cuboid after SLA printing (right) and final sintering(left) to visualize the overall volume shrinkage.

D Determination of Volume Shrinkage

A sol containing about 40 wt.-% zirconia nanoparticles, acrylatecomponents and photoinitiator was moulded in a rectangular mould andlight cured. The sample had a dimension of 75*44*35 mm.

The sample was super critically dried as described above and measuredagain. The shrinkage in this step is mainly determined by the amount andnature of the matrix components (acrylates).

In a next step the organic components were burned out and the samplepre-sintered at 1035° C. for 70 h. The shrinkage in this step is mainlyregulated by the organic content, the content of zirconia and thehomogeneity of the green body. In this step the main shrinkage takesplace.

In a final step the sample was sintered at 1300° C. for 2 h until finaldensity was received.

The final shrinkage overall is about 53% linear or about 90% in volume,respectively. The values of volume shrinkage, linear shrinkage anddensity are listed in Table 2. The relative volume and relative lengthare calculated based on the Volume F being scaled to 100.

TABLE 2 Shrinkage Shrinkage Process Volume Vol. Relative Length LinearRelative Density step [mm³] [%] volume [mm] [%] length [g/cm³] Gel body115,500 — 1004 75 — 214 0.15 (Volume A) After super 86,900 25 756 69  8197 1 critical drying (Volume C) After organic 24,200 79 210 45 40 1292.9 burnout and pre-sintering (Volume E) Final sintering 11,500 90 10035 53 100 6.05 (Volume F)

What is claimed is:
 1. A process for producing a ceramic article, theprocess comprising: providing a printing sol, wherein the printing solcomprising solvent, nano-sized particles, radiation curable monomer(s)and photoinitiator, and wherein the printing sol having a viscosity ofless than 500 mPa*s at 23° C.; processing the printing sol asconstruction material in an additive manufacturing process to obtain a3-dimensional article being in a gel state, the 3-dimensional articlehaving a Volume A, transferring the 3-dimensional article being in a gelstate to a 3-dimensional article being in a dry state, namely an aerogelor xerogel; and applying a heat treatment step to obtain a sintered 3ceramic article, the ceramic article having a Volume F; wherein Volume Aof the 3-dimensional article in a gel state being more than 500% ofVolume F of the ceramic article in its sintered state.
 2. The process ofclaim 1, the transfer of the 3-dimensional article being in a gel stateto a 3-dimensional article being in a dry state being conducted byapplying a supercritical drying step.
 3. The process of claim 1, theadditive manufacturing process being selected from: stereolithographicprinting.
 4. The process of claim 1, the additive manufacturingprocessing comprising: providing a layer of the construction material ona surface; radiation curing those parts of the layer of constructionmaterial which will belong to the 3-dimensional article to be produced;providing an additional layer of the construction material in contactwith the radiation cured surface of the previous layer; and repeatingthe previous steps until the 3-dimensional article is obtained.
 5. Theprocess of claim 1, the process not comprising either or all of thefollowing steps: heating the construction material during the additivemanufacturing processing step to a temperature above 70° C.; applyingpressure during the sintering step.
 6. The process of claim 1, theprinting sol being characterized by at least one or all of the followingfeatures: being translucent in a wavelength range from 420 to 600 nm fora path length of 10 mm; showing a transmission of at least 5% at 420 nmdetermined for a path length of 10 mm; pH value: from 1 to
 6. 7. Theprocess of claim 1, the printing sol being characterized by at least oneor more or all of the following features: the nano-sized particles beingpresent in an amount from 20 to 70 wt.-% with respect to the weight ofthe printing sol; the average primary particle size of the nano-sizedzirconia particles being in a range up to 50 nm.
 8. The process of claim1, the nano-sized particles being characterized by at least one or allof the following features: being crystalline; being essentiallyspherical, cuboidal or a mixture thereof; being non-associated;comprising ZrO₂ in an amount of 70 to 100 mol-%; comprising HfO₂ in anamount of 0 to 4.5 mol-%; comprising a stabilizer selected from Y₂O₃,CeO₂, MgO, CaO, La₂O₃ or a combination thereof in an amount of 0 to 30mol-%; comprising Al₂O₃ in an amount of 0 to 1 mol-%.
 9. The process ofclaim 1, the printing sol comprising in addition organic dye(s).
 10. Theprocess of claim 9, the organic dye being characterized by at least oneor all of the following features: being present in an amount from 0.001to 0.5 wt.-% with respect to the weight of the printing sol; showing aradiation absorption in the range from 200 to 500 nm; having a molecularweight in the range of 100 to 1,000 g/mol; being soluble in the solvent;being combustible without residues at a temperature below 800° C.; notcontaining heavy metal ions with an atomic mass above
 40. 11. Theprocess of claim 1, the printing sol in addition comprisinginhibitor(s), preferably in an amount from 0.001 to 1 wt.-% with respectto the weight of the printing sol.
 12. The process of claim 1, theradiation curable monomer(s) being characterized by at least one or allof the following features: being represented by a formula A-B with Acomprising an acidic group, a silane group, an amine, amide or alcoholgroup and B comprising a vinyl group; being present in the printing solin an amount from 2 to 30 wt.-% with respect to the weight of theprinting sol.
 13. The process of claim 1, the printing sol notcomprising either or all of the following components: wax(es) in anamount of more than 0.1 wt.-%; insoluble pigment(s) in an amount of morethan 0.5 wt.-%; particles having a primary particle diameter larger than100 nm in an amount of more than 1 wt.-%; wt.-% with respect to theweight of the printing sol.
 14. A ceramic article obtained by theprocess of claim 1, the ceramic article being characterized by either orall of the following features: density: more than 98.5% with respect totheoretical density; translucency: more than 30% determined on apolished sample having a thickness of 1 mm; flexural strength: at least450 MPa according to ISO 6872; phase content tetragonal phase: from 0 to100 wt.-%; phase content cubic phase: from 0 to 100 wt.-%; dimension ineither x, y or z direction: at least 0.25 mm.
 15. The ceramic article ofclaim 14, the ceramic article having the shape of a dental ororthodontic article.